Walk down the stairs, hang a left and keep going forwards until you're peering over the edge and looking down at the platform with the red switch on it. Get a portal on the yellow/black coloured panel and then fire a portal under the blue gel dispenser.
Keeping one portal under the gel dispenser, place the second one on the slanted roof up in the right-hand corner of the room. Once enough blue gel has passed through the portals, place a portal on the vertical wall just below the angled wall and then directly under the orange gel dispenser.
Step through the portal and you’ll arrive on the suspended platform covered in orange propulsion gel. Run to the end of the platform and then turn around to face the two wall panels. Get a portal onto the middle of each one and then run through it, you’ll land on the blue gel, bouncing you over to the platform suspended in the air.
Pick up the Wooden Test Cube before dropping down to the platform below, once down here, set a portal on the wall to your left and then zoom in to the wall just to your right (in the distance). Place your next portal here and carry the Cube over to the red square button on the floor. This'll rotate the panel so it's facing upwards.
Now zoom in and look for a platform suspended up and to your left in the distance, there’s a portion of wall on the right side of it you need to place a portal on, so step through the portal behind you and you’ll arrive up on the new platform.
Note the ramp leading from the wall and get a portal on the wall facing the ramp. Now turn around to your right and set another portal on the panel under the orange gel dispenser. This'll coat the small ramp with orange gel.
Now turn to your left and set an exit portal on the vertical wall where you’ve already coated the floor with orange propulsion gel. Step through the portal, run to the end of the platform, turn around and then run through the portals, this’ll give you enough speed to land on the overturned blue gel, bouncing you straight over to the exit!