This achievement is done on Chapter 2 on the 6th test chamber with all of the aerial faith plates. Go through all of the plates until you are on a platform with the button on it. When you press it the first time GLaDOS will shoot trash at you. In the junk, pick up the radio and the go back to the beginning of the chamber. Now place a portal on the wall to the right of the angled platforms (see video below) and you will be launched into Rat Man's den. Once you are fully in the room the radio will play some weird noised and you will get the achievement.
All credit for the video goes to Rooster Teeth: