In the story on the level Crows Nest, at the start of the level, the skull is above you on a pipe (in ops centre).
You jump up in some shelves the on to a red gurder and then a big pipe and its at the end.
When you've got it press RT shoot button.
Hint: if you keep pressing it he will play basket and if on 2 player go close to the other person and they will pass the skull. You've got to be close or he'll keep bouncing it up and down.