After GlaDOS's predictably fake surprise, go through the offices to find a large test chamber. There's a turret at the far wall, so be careful not to let it shoot you. To the left there is a laser coming down from the ceiling. There are several laser relays on the floor. A light bridge is going from the floor to the ceiling. There is a glass wall in the middle of the floor, and a laser cube to the right. The floor is surrounded by toxic sludge.
Put a portal on the far wall where there is a platform and some glass. Then put a portal of the other color that you can walk into. Go through to reach the far platform. There is a small button here. When you press it, two laser cubes fall from the two chutes and land on catapults. However, they both fall into the toxic sludge and are destroyed. You can press the small button to make more cubes fall. But you have to stop the cubes so they won't fall into the sludge.
To do that, create a portal on the ceiling where the light bridge is shining. Create another portal on the floor near the far cube chute such that the cube will hit the light bridge when it gets catapulted. Then press the small button. With luck, both cubes will end up on the floor near where you entered the room.
Create a portal on the back wall near you, then create one over where the cubes fell. Retrieve the cubes, being careful to avoid turret fire.
Create a portal on the wall near the laser, and a portal of the other color near you. Take a cube over to the laser and put it into the path of the laser. Make the laser shine on a white wall. Use portals to go back to where the other laser cube is. Then create a portal on the wall where the laser is shining. This way you can make a laser come out of a portal near you. Put the second laser cube in the path of the laser and use it to destroy the turret.
Go back over to the laser cube that is in the path of the ceiling laser. Turn the cube so the laser is shining through the two laser relays that you can shine on while standing near the place where the laser is coming down from the ceiling.
Now go get the second laser cube. Go past where the laser is going through the two relays, then put the laser cube in the path of the laser such that it's not blocking the two illuminated relays. Use this second laser cube to light up the two remaining relays as well as the laser catcher near the exit door.
To reach the exit, make a portal on the ceiling where the light bridge is shining, then create a portal of the other color on the white wall that is on the other side of the area where the laser is shining from the ceiling. That way you can walk along the light bridge to reach the exit. Be careful not to fall into the toxic sludge or get lasered to death.