Just go forward and step on the catapult, then sit back and wait for the next chapter.

You'll end up in front of an array of Wheatley monitors. He'll reveal his big surprise: big smashy spiky plates. But there's some conversion gel dripping on a wall below the Wheatley monitors. Put a portal there, then another portal on the white wall to the left of the monitors. The gel will drip onto the platform that you are standing on. Look at the portal on the wall to the left of the monitors, then put a portal below you of the other color. You'll end up on the walkway behind the monitors and can make your escape.

When you reach the end of the walkway, put a portal on the floor down there, then look for another walkway to the left, then look for the white wall facing that walkway. Put a portal of the other color on there, then drop into the portal down below. You should end up on the other walkway.

Keep going and you'll see a frankencube, which Wheatley will cruelly smash into the wall (although the cube seems fine afterward, at least). Put a portal on the wall above the other side of the walkway, then go back to the archway/door that you passed through a moment ago and put a portal of the other color on one of the walls there. Carefully step through to land on the other side of the walkway. Go through the emancipation grill and go to the exit door.