In a good, clearly explained short video, Peggi Kroll-Roberts Workshop: Painting the Head, painter Peggi Kroll-Roberts explains how to find the proportions and features of the head, with the face looking forward.
She talks about seeing the head as an egg or a cube, in terms of its general shapes. Next you find the spot halfway, lengthwise, between the top of the head and the chin — that gives you the location of the eyes. The eyes are generally one eye-width apart.
She next locates the hairline, and divides the lengthwise space between it and the chin in thirds. Those thirds give you the placement of the eyebrows and the bottom of the nose.
Finally, she draws a small horizontal line halfway between the bottom of the nose and the chin. That serves not as the placement of the mouth, but rather the place on which the mouth rests.
Having located the features, Kroll-Roberts then dilineates the planes of the face and head — first the large essential shapes on the right (above), and then the smaller planes into which the head can be further divided. Knowing these shapes will help painters not only paint them, but also understand how the light and shadow will fall on the face's features.
You can see more of Peggi Kroll-Roberts's artwork at www.krollroberts.com.