You will reach a point where you use an elevator for the first time after meeting a ghost girl called Sadness. Pick floor 1, then hold B to jump out of your body, your achievement tracker will go up. You will get this over natural progress of the game but you can do it here if you just want to get it over with. You will die if you let it continue for over a minute (10% in the tracker), so hold B again to go back to your body. Rinse and repeat.