My drawing above is copied from "Drawing from Life" by Brown and McLean. And while I learned a lot of technical info by copying from the book, a lot of the problems are already figured out for you if you are copying a 2D source to a 2D drawing. To really challenge yourself and to fully understand the shapes of the skull, I recommend you
buy a plastic skull. (This is not only a great investment in your artistic endeavors. It's awesome to put in the window for Halloween!)
Set up a simple still-life of the plastic skull and draw it from many vantage points. Get to know the landmarks of the zygomatic arch, the orbital bone, and the mandible. See how angles change and intersect. Study the size and shape of the whole skull.