Story related so you can't miss it.
SPOILERS AHEAD
After you've dealt with the turret factory and the neurotoxin generators you'll be sucked through the pneumatic diversity vent to the entrance to GLaDOS's chamber. Walk up the stairs and you can note that the bridge leading across to her chamber from the first game is destroyed. Stand on the edge of the pit and look off to your right. In the distance there will be a small square of tile you can portal onto. Head over there and approach the door labeled something like "GLaDOS emergency shutdown and cake dispensary".
When you get to her chamber just let her talk until Wheatley shows up. Pick him up and bring him to the center of the room and insert him in the receptacle when it rises from the floor. There will be a short dialogue after which you will need to go push a button. GLaDOS has panels that will fling you away if you try to walk to it though so portal over to it and portal to the opposite side of that room to get past the panels that are close to the button. Once you push the button return to the center of the chamber and watch the fun unfold until the achievement pops.