The first place in the story you can earn this is Test Chamber 2-1. However, you can get it earlier. This is easily done in the Level Editor.
Go to the level editor and create a new level. Create a floor and an entrance and exit door with a way to trigger exit door (like a terminal).
The important part is the entrance door. I could not find an 'Inflate-A-Mate' in the menu with the objects you can place. You actually need to 'edit' the entrance door to have an 'Inflate-A-Mate'. Once done, load play the level, deploy the Inflate-A-Mate by left-clicking then immediately right clicking to inflate it. Make sure you are under it so it crushes you and enjoy your achievement!