I seem to be stuck on ending with Xana and taking the skull for myself. I have picked up the skull and it's in my inventory. Xana keeps saying 'use the skull'. So how do I 'use' the skull to end the game with Xana and me with the skull ie I rule the world? Driving me crazy!

Put it back to where you got it from = keeping it.
Or walk up the stairs and throw it into the hole = handing it over to "Father".