Before You Begin
You must play the game on a difficulty of Normal or higher. Start at the beginning of the level the skull is on. Successfully save and quit to acquire the skull once you've found it. Some skulls do not indicate that you have retrieved them. However, if you are holding a skull that means you have gotten it. Refer to your skull collection in the pause menu by pressing "X."
Skulls Are Hard to Find
All of the skulls are hidden purposefully. They will take some effort to find; see the link in References for specific locations. Also, there will be no indication of how to pick up the skull. You must press "Reload" to pick up the skull.
Clear the Area
Clearing the area you know the skull is in is essential to unhindered skull collecting. If the area the skull is in is still swarming with enemies, they will make it difficult to retrieve the skull, especially since the difficulty itself will be higher than easy. Also, make sure you have full health before hunting.
Ditch the Skull
Unlike in "Halo 2," you do not need to carry the skull to a save point for it to be active in your profile. After having your hands on the skull, you can ditch it and continue playing, or save and exit the game. the skull will still be in your inventory and you won't have to play through the whole level.