Test Chamber 1
Player 1: As soon as the chamber begins, go left and place a portal on the side of the ledge, then on the only wall panel located slightly higher on the left wall. Once you stand next to the red button, press it to drop some repulsion gel on the floor near the elevator.
Player 2: Find the large red square switch in the center of the room and step on it. The elevator will drop near the first floor. Don't leave the switch until your partner jumps on the elevator.
Player 1: Jump off the ledge, off the repulsion gel, onto the elevator, and once you reach the top floor, press the red button to start a new flow of repulsion gel next to the elevator on the top ledge. Now jump back down to the first floor, return to your original two portals, and you should now notice a lowered elevator ahead. Jump off the repulsion gel and climb onto the elevator. Have Player 2 step off the button to raise the elevator you're on. Once you're there, find the panel encased in the wall, place a portal so your friend can slip through the lower portal and join you at the exit!
Test Chamber 2
Player 1: There's a slanted wall panel in the upper right corner by the force field, so place a portal there, then just under the repulsion gel dispenser to your right. Press the red button, and the gel will cover the floor on the left side.
Player 2: Jump off the repulsion gel—through the force field—and, once outside, turn left toward the excursion funnel. Place a portal on the panel here and then look down through the ledge toward the two horizontal floor platforms. You want to place the vertical excursion funnel on the platform furthest from you.
Player 1: There's a gap in the force field under the inclined panel, so zoom in to the single panel on the opposite wall (near the funnel) and place a portal there. Now press the red button to make the funnel catch the gel.
Player 2: Once the gel hits the hopper, place the hopper in a different location so the gel falls straight down and covers the platform. Now look at the first platform, and there's a shooting pad on the wall to the left. Place a horizontal hopper there so it's directly above the platform.
Player 1: Press the red button to coat the nearby platform with repulsion gel, then return to Player 2 outside. Stand on the square red button to extend the ledge near the exit on the other side of the field.
Player 2: Run and jump from the ledge to the first platform below, jump to the second platform, and you should land right next to the exit. Now look to the right: there's a wall panel here where you need to place the excursion funnel so it moves horizontally directly beneath you.
Player 1: Now it's your turn to jump from the ledge across the platforms below. Once you hit the excursion funnel, jump onto the walkway and join Player 2 at the exit.
Test Chamber 3
Player 1: Immediately after entering the Test Chamber correctly, turn left, and you should see a light bridge. Place a portal on the wall here, and then place your second portal on the leftmost panel, located directly below the "EXIT" sign painted on the wall in the distance, creating a bridge that extends across the entire area.
Player 2: Join Player 1 on the light bridge and once you are near the red button (located on your right) you need to place a portal on the end of the light bridge and then on the left wall so that the bridge leads to the right to the red button.
Player 1: Press the red button, and if you placed the light bridge correctly, the repulsion gel will land on the bridge, allowing you both to jump to the other side and the second part of the chamber.
Part 2
Player 1: Once you're on the light bridge, turn left and place a portal on both panels at the bottom. This will allow Player 2 to cross to the other side, bypassing the force field. While you're preparing, stand next to the red button and get ready to press it.
Player 2: Descend the light bridge and place portals on the two panels in the corner of the room. When the two light bridges are ready, notify your teammate.
Player 1: Press the red button to release repulsion gel onto Player 2's light bridge. Tell them to jump off the ledge ahead.
Player 2: Once you're on the ledge, turn around and place a portal on the single panel directly below the repulsion gel dispenser. Now run back to where your buddy is and look for the single horizontal panel hanging above the main light bridge. Create a portal and have Player 1 press the red button again.
Player 1: Now that the light bridge is coated with gel, use it to jump up onto the ledge with Player 2 and create a path to the exit.
Test Chamber 4
Player 1: Your first task is to place a portal on the bottom panel on the left, and then on the slanted panel in the corner. Press the red plunger, and the repulsion gel will cover the left vertical panel leading upward (to be absolutely sure you've covered the wall panel enough, press the plunger twice). Now look straight up at the ceiling, and you should notice a second slanted panel pointing towards the right vertical panels. Place your exit portal here and press the red button twice again to completely cover this wall with gel.
Player 2: There's a floor panel on the right side of the room that you can shoot, so place a portal there and then on any of the panels just above. This will leave you standing on the upper ledge. Now have a friend place a portal on the floor (above you) and on the slanted panel at ground level. Jump through the portals, and you'll bounce to the exit above.
Player 1: Stand on the platform Player 2 was on and have your colleague place a portal on the floor below you and on the slanted panel nearby. Jump and bounce off each side of the gel-covered panels (make sure you align correctly or you'll bounce off the wall and have to start over)!
Part 2
Player 2: Once you get close to the permanent light bridge, look at the panel where it extends and place a panel there, now place an exit portal on the panel in the top left corner of the area (opposite the force field).
Player 1: Ride the light bridge across the force field and when you reach the other side, find the floor panel just before where the light bridge ends (directly below the repulsion gel dispenser). Place a portal here, then look directly above you at the two slanted wall panels, place a portal on either slanted panel, and have Player 2 press the red button.
This will cover the first side of the walls. Now place the portal on the other panel and have Player 2 press the button again so the next side is completely covered in gel as well. Just like before, you need to bounce up the walls Samus Aran style until you reach the top. Turn around and look down at the floor panel again—under the gel dispenser—and return the portal there.
Look over the edge of the platform, and if you look straight ahead, there's a slanted panel behind the metal fence; this is where you want your second portal to go. Now have Player 2 press the red button to release another burst of repulsion gel.
This will cover a long panel that won't be visible in front of you. Once the wall is sufficiently covered, look straight up to see two more slanted panels. You want the portal to be on the right panel.
Player 2: Remove the portal at the light bridge and place the Exit Portal on the other slanted panel next to Player 1. This will create a wall adjacent to the gel-covered wall, creating the perfect path to cross to the exit.
Press the red gel release button again, and the gel will now coat the vertical bridge, allowing you to wall-jump up and to the right with your friend.
Test Chamber 5
Player 1: Upon entering the test chamber, turn left, placing your first portal on the panel directly across from the elevated platform, and your second portal on the floor panel under the propulsion gel dispenser. Press the button, and propulsion gel will coat the inclined platform in preparation for the next steps.
Player 2: Run along the orange runway, and you'll fly over the gap to the other side. Once you're safely across, stand by the red button and begin the countdown: 3, 2, 1.
Player 1: Immediately after the countdown begins, start running up the ramp and, just before reaching the force field, Player 2 must press the button to release the crate above. As you fly past, press the grab button to catch the crate and place it on the square button on the other side. Both players: Pass through the force field, descend the stairwells, and enter the second part of the test chamber.
Part 2
Player 1: Drop down to the lower level and place a portal on the left-hand inclined wall panel (marked "2" on it), placing the next portal directly under the Propulsion Gel dispenser on the right. Press the button and watch as the gel covers the floor where the metal spikes fall.
It will take a few tries to cover the floor enough as the spikes constantly rise and fall (blocking the gel). Repeat the same steps with the inclined panel next to you (marked "1").
The next step is to leave the portal under the gel dispenser, but place another portal on various panels on the upper ledge (including all the side panels and roof panels). This should evenly cover the area of the floor ledges.
Player 2: Stand on the left ledge and place a portal on the wall behind you and on the wall in front of you to create an "infinite" portal. Now run and don't stop!
Player 1: Once Player 2 starts looping, place a portal on the slanted panel "2", aim your portal gun at the right wall, and fire the second portal as soon as they pass through their portal to the right, which will allow you to shoot through the spike plates. Ideally, you should listen for the sound of the portal before firing the second portal.
Once they've safely passed the spike plates, move to the same platform your friend was on and create your own infinite portal (one portal behind you and one in front of you). Start running, and you're now in your colleague's hands.
Player 2: Place a portal on the inclined panel "1," then time your shot at the right wall so the spikes are just beginning to rise again. If timed correctly, Player 1 will fly into the air and avoid the spikes. Both players: Step on the red switches simultaneously to release the crate from the chute in the center of the room. Pick it up and throw it at the lock on the floor to open the exit to Chamber 6.
Test Chamber 6
Player 1: Look for the red laser in the distance and south of it is a inclined panel. Place your first portal here, then place your second portal directly under the Propulsion Gel dispenser. Press the button twice and it will cover the inclined platform in the center of the room.
Player 2: Once the platform is completely covered, use your portals to reach here, then stand there, preparing for Player 1's approaching countdown.
Player 1: Stand near the red button and look straight ahead. In the distance, you should see a panel with a red arrow pointing downwards (near the water). Replace the portal from the dispenser here and stand near the red button.
You'll need to be extremely quick in the next few steps if you're going to make it. So, start the countdown timer, and once it reaches "2," tell Player 2 to start running and press the red button to release the Displacement Cube into the portals. If timed correctly, Player 2 will catch the box in mid-air as it rushes toward the portal, from which the Displacement Cube will emerge.
Now you need to be especially quick. Once your colleague catches the Cube, look toward the water and find two square panels. You want to place the exit portal on either of the two panels; this will force Player 2 to swim directly toward the red laser beam.
Player 2: Your goal is to aim the Cube at the turrets directly in front of you so that the beam is deflected and they all light up!
Player 1: Before Player 2 drops back down the portal they came from, place a second portal on another square panel so that your friend flies back into the laser beam, allowing them to redirect the laser beam to the second set of turrets. The next task is to turn left and find the corner panel and place the exit portal there. If you time it correctly, your teammate will fly towards the exit. Once they do, move onto the long, sloping platform covered in orange propulsion gel and race up the ramp to the sloping panel ahead.
Player 2: The goal is to place a portal on the wall Player 1 is about to crash into, and another one on the sloping panel you last came out of to shoot. Get it right, and Player 1 will join you at the exit.
Test Chamber 7
Player 1: This is a nice and easy challenge before the final co-op chamber. As soon as you enter the room, look to the left at the small rectangular area with a ceiling and walls to shoot at (but no floor). Place a portal in the ceiling section, go to the Propulsion Gel dispenser, place a portal there, and use the button to release the gel. You need to move the portal halfway so it covers the entire floor.
Player 2: Once there's enough gel on the floor, create an "infinite loop" portal and keep running!
Player 1: Place a portal on the corner panel located up and to your right, run down to the end of the chamber with two red switches. Press the one directly in front of you and quickly run back a bit. Now shoot the portal near the right wall of Player 2's loop, and he will be ejected at high speed onto the platform you made by pressing the button earlier.
Player 2: Run to the end and have Player 1 quickly press the other red button to release the crate from the dispenser above you. Catch the cube, then run back as quickly as possible (as the platforms are on a short 15-second timer)! Once you're on the ground, simply throw the crate at the switch location to open the exit to the final chamber.
Test Chamber 8
Player 1: You need to act quickly here, but as soon as the door opens, immediately run to the red button on the right and press it. This will drop a blue repulsion gel on top of the turret, killing it (back away, as you can be killed by its bullets).
Player 2: Once the turret is taken care of, use the blue gel to bounce to the other side of the room and set up a portal on the inclined panel and directly under the propulsion gel dispenser.
Player 1: Place a portal on the slanted panel outside (opposite the force field), then on the panel directly above the entrance (ideally, about halfway up). Press the red button, and propulsion gel will flow through all four portals, covering the ground in front of you.
Have Player 2 join you outside, and both of you simply need to run along the gel and press jump when you hit the blue gel to propel yourself across the gap to the passage on the other side.
Once you reach the other side, pass through the force field and immediately turn left. Look straight ahead at the end of the path and shoot through the portal into a cleverly hidden wall panel.
Player 2: There's a pillar near the end of the passage where Player 1 is now, so place your first portal on it before moving slightly to the right and placing another portal on the wall across the gap. This will allow Player 1 to pass through.
Player 1: Go through the friend portal and head down the stairs a bit. Now look over the railing to your left, and there's another panel below you you can use. Set up your second portal here and head back to the edge of the walkway.
Both players: If you both exit the walkway and enter the portal below, you'll dash across the gap and head to the second—and most challenging—part of this final test chamber.
Part 2
Player 1: Take the right side first and use your portals to cross the gap to the other side. Once you're through the force field, head straight to the red switch and set up a portal under the gel dispenser, then turn right and look for a panel on the wall next to the force field. Set up your second portal here and hit the switch to get the gel right in the corner of the room.
Player 2: Set up a portal on the far left corner of the wall, then another one under the gel dispenser and hit the button, this will create a nice runway you can run up. Place a portal in the checkered floor tiles at the far end of the "runway" and rush through it.
When you reach the peak of your climb, fire a portal at the wall in the distance in front of you. This will allow both of you to easily reach the upper level. For now, simply drop down to the lower floor and stand by the red button.
Player 1: Go through your friends' portals, and when you reach the upper level, look over the ledge to the ground below and place a portal under the dispenser. Look up and to the left to see a panel directly across from the red switch. Place an exit portal here, press the red button next to you, and have Player 2 press the button to release the gel.
This will coat the crate you released in propulsion gel, allowing it to glide right towards you! Just before you lift it, look across the gap ahead and there are a couple of panels on the right wall that can be used. Place a portal here, then on the wall behind you, grab the slime crate and carry it through.
On the other side, you should see an orange arrow pointing forward. If you look through the gap, you'll spot a section of wall a short distance away that you can shoot. Create a portal there, grab the crate, and carry it through. Now stand at the top of the hill where the laser beams are shining, but DO NOT let go of the crate!
Player 2: Follow your teammate until you see the orange arrow, but stay here! If you look to the right, you'll notice a pair of switches on the wall in the distance. That's your goal.
Player 1: Once Player 2 is in position, throw the crate halfway down the slope and immediately run back through the portals and make a mad dash across the lifting platforms, each of you needing to reach a switch. Now, before the platforms give way, press the switches together to release the first set of gels leading to the exit.
Once you both touch the ground, use your portals to reach the other side of the gap in front of you and wait for Player 2 before you pass through the force field into the final series of challenges.
Player 2: Upon entering the next area, look to the right and place a portal on the ground near the excursion vortex. Then, walk onto the vertical square panel directly above the entrance and step on the red square switch on the floor (this switch rotates the panel on the other side of the force field, so don't move)!
Player 1: Look up at the ceiling and find a portal on the panel. Now place the next portal under the repulsion gel dispenser and press the red button to release the gel. The excursion vortex will catch the gel and carry it through the force field. Walk through the force field and quickly shoot the rotated panel the vortex hits, placing an exit portal on the panel in the upper right corner.
Wait until the gel is directly above several turrets before retreating through the force field to cut the vortex, dropping the gel on the turrets, killing them all! After clearing out those pesky turrets, use the gel-covered walls to jump across the gap.
Once you're on the other side, have Player 2 join you, and you both need to jump up to where the towers used to be. The goal is to shoot the portal on the floor that isn't covered in blue gel, and then reactivate the portal on the panel directly above it.
Player 2: Your goal is to stand on the red switch and reset the portal on the excursion vortex, then on the rotated panel directly above you on the wall. This will create a vortex that will extend through your head.
Player 1: Jump through the portals created and the speed and height will take you to the excursion funnel above Player 2. Ride to the end, exit, press the red button here (to release the blue repulsion gel inside the main area) and simply jump down and you both will need to get back outside to the main area.
Both players: Run down the propulsion gel, bounce off it, and you'll land at the large vault door. BOTH of you must perform any gesture in front of the camera to complete the co-op move and trigger the ending sequence! Congratulations!
Portal 2: Co-op Walkthrough:
Course 5: Acceleration Gels
Source: gamerguides.com