Skulls, my friend. Learn how to draw skulls, and the rest will come easy.
Look at a lot of reference pictures, keep drawing every detail you can in full detail until you got it memorized, then try drawing a face over it. The skin and muscles around our skull is very thin, apart from things like our lips, noses, and ears it's all pretty taut over the form of the skull, so it's necessary to really drill that underlying structure into your head.
Google image searches are your friend, work from reference wherever possible. Be sure you're looking at actual skulls and not a render too, sometimes the modelers can make mistakes.