Tips that I learn to drawing a three-dimensional head: add value (shading); two sides are symmetrical; widest part of the head is just above the ear and marks the top half bottom half; eye socket area angles down; the largest plane of the face includes cheeks, bisecting nose, temple to temple, eye socket to the upper lip-like a triangle; nasal bone is a pyramid; and the side planes reach to the back of the head and include the cheek/jaw bone area.
Below are pictures I drew based on the portraits at the gallery.
