Chapter 2, Chamber 6.
After pressing the button for the first time, a turret should end up on the platform between the two main surfaces. Take this turret from the small platform to the beginning of a level (through a portal) and put it on the pusher plate at the start. Achievement unlocked!
Like stated in the video, there are other chances within the game to get this achievement, like chapter 3, chamber 15.
Note: If the turret is not in between the platforms nor at the end of the level, it may have fallen into the goo. Just restart the level, and keep track of it coming out of the disposal.