How do you activate skulls in Halo 3?

Once you get a skull from a mission, you can activate it from the campaign menu. In the Campaign menu, click X and you will see a whole bunch of skulls. The ones that are brighter are the ones you have collected. Highlight one of the skulls you have collected and click A to activate it for the missions you will do. Skulls are only useful for Campaign scoring to multiply the amount of points you earn, though.