Conditioned clay is applied to the skull to a thickness of about 1/8" thick. The eye sockets are kept open at this point and the main features are roughed-in.
The features are refined slightly as the upper lip is attached.
The lower lip is attached, features are refined a bit more and beads are dropped into the eye sockets.
Using photos, the features are shaped and refined. You will get a better result by working all the features at once rather than concentrating on refining just one feature at a time.