Location: After encountering several Prowlers, move through the desert and eventually you'll fall down a small cliff. At that point, turn into the area off to the right. After turning right, drive beneath the giant walkway. At the very end of the walkway, get out and jump up onto the big boulders next to it. From atop the boulders, jump up onto the walkway. You need a Plasma Grenade for this next part. If you don't have one, it is probable that one of the Ghost-riding Grunts in this area dropped one or more Plasma Grenades.
Now for the tough part. First, move toward the support at the very end of the walkway. The ‘Famine' skull is at the very end of the space in the middle of the support. To reach the skull, you'll need to use a grenade to propel yourself to this space. To do this, place a Plasma Grenade down in front of you (near the edge of the walkway, just before the flat part), and then run and jump past the grenade. If done correctly, the explosion should be enough to propel you to the empty space. If you don't make it, press start and select “Revert to Last Save” to try again (only if you activated the checkpoint in this area by destroying the Ghosts). This video is from our unofficial game guide. You can view the full guide here