How do you unlock Skulls in Halo 3?

[AnimeAddict231]

I've found a skull, now what do I do with it?
I have found a skull in a level, lets say the Blind Skull in Sierra 117, do i have to finish the level with it in my hands in order to unlock it, or what. what do i do with it in-level?

 

[EZ492]

well to use them press x while in the campaign lobby or if your talking about the finding them go on youtube you have to find them youtube will help