I forget exactly what the door looks like. You've come around the corridor with the cells and skeletons on your left and you've turned into a room on the right. Ahead of you is a door with...skulls?...well, something on it. You want to put the skull on the floor in front of that. Just move it slowly over the floor and you'll get one of those glowing green indicators.
"Best not to think about it. I don't want to fall to bits 'cos of excess existential thought."