Instructions
Play the game until you complete the "Mad Doctor (Part I)" 2-D area. You will find yourself in the foyer of the Manor House.
Go to the main room of the foyer and you will find painted walls on either side of the room. Spray these walls with paint thinner to remove the paint.
Go through the painted wall on one side and travel down the hall to the hidden room. Once in the hidden room, use paint on the skull to make it fully appear.
Repeat this process with the wall on the other side of the room. Once the two skulls are fully painted, the path into the manor will open.