Create a portal on the wall that the light bridge is shining on. Approach the exit door and create a portal of the other color on the wall to the left of the exit door. This creates a bridge that is coming out of the wall and goes under the cube chute near where you first entered the room.
Go through the portal to walk along the light bridge. After going through the portal, look for the small button to your right. Press it to make a weighted cube come out of the chute. Now walk toward it.
Surprise! The lights go out, the bridge disappears under you, and suddenly Wheatley is there. His bad American accent doesn't fool GlaDOS. So now you have to run.
Wheatley opens the wall, so go through. A light bridge appears under you. Walk across it. It drops you off on a walkway. Turn left and follow the path. Avoid dead ends. When you see some stairs, go up. At the next two forks in the path, go straight.
Soon enough, a new light bridge appears and GlaDOS encourages you to do the final test that she had planned. Don't do it. Go the other way. The light bridge will disappear under you and you'll land on another walkway. Turn around and continue along the path. Soon there will be a loading screen break.
After that, keep following Wheatley. You end up in an area with a low ceiling. The walls will close in and turrets will be placed in the area. Knock them down while hiding behind the tubes. Then look for a gap in the wall through which you can see a white portal wall outside of your enclosed area. Then create a portal on the white wall that is within your enclosed area. Go through.
Follow Wheatley. Soon enough a turret will be deployed. Check which color of portal you came out of when you escaped from the enclosed area, then carefully look into the area with the new turret and create a portal of the other color on the floor under it to get rid of it. Now you can safely go up the stairs.
Follow Wheatley. When the place starts collapsing, go straight ahead toward the wall where there is an elevator. Go into it. When it reaches the top, go through the door up there and walk straight until you encounter another loading screen.