This achievement is done in Chapter 4 after 3 puzzles when Wheatley stops the 4th test to rescue you. Just a little bit into the scene, GLaDOS asks for you to complete the "final" test and a platform appears or you to go over to it. Simply listen to her, go over to the testing room and the achievement pops once you are stuck there.
Here is a video I found that shows this. All credit for the video goes to stangdriv3r.