The ball or back of the head shape is the cranial mass, and the triangular shape formed by drawing a line from the bridge of the nose to the jaw hinge is the facial mass. There is a lot to learn about head structures, but in my journey I wanted to know the basics and find easy ways to represent them for correctly proportioned portraits. I simply use a circle to represent the cranial mass and an oval to represent the facial mass.
Often when drawing faces in mixed media, artists tend to treat the whole head as one shape, usually an egg shape, rather than two separate shapes, which is a more realistic approach. Treating the head mass as a sphere and an oval helps you create a head rather than just a face and adds so much more dimension to your work. It’s not as complicated as it seems, and you don’t need to get into great detail. Remembering mass and working around it will help you create more realistic faces.
Using a Styrofoam wig form and light can help you capture the planes of the face with the light coming from different sources. It’s an inexpensive tool and can easily be found online or at a beauty supply store. Just set the wig form under a light and study how the light hits the planes of the face.
This sketch shows the head mass or ball of the head and some simple face planes from a side view.