Well, it's hard, I certainly haven't mastered it, but a lot can be learned just by getting good pictures or models and measuring as you go. Compare the length of a part to another part, and be sure to work from these throughout, maintaining proportion (and scale) in the body. For example, the length from the hairline to the eyebrows, and how it compares to the distance from the nose to the chin, and how yet another lengthwise measurement compares to the width of the head at a certain point. Do all the mapping, then overlay with tracing paper and work out any further problems, back away and check, look at it in a mirror and see if it still looks natural. If it's right, it should. If it looks like it has depth and everything is where it belongs. Because you can learn statistics only so much; humans vary in proportion just enough to throw things off. I have a high forehead and one eye just slightly more open than the other, and the curve of one cheek is a little sharper than the other, for example.
Even when trying to get clear proportion, remember to start with the large shapes. Get them roughed out to help get the position of the form, then work and adjust. And that tracing paper thing works for any drawing, helping you keep from getting it too set before having the problems solved. Keeps you from starting on small details too soon. Once you have the drawing you want, you rub pencil or whatever on the back, tape it over the final surface, and transfer it.
Of course, you may have heard all this already or think it's not quite what you wanted, but it helps me a lot.
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"Pack up my head, I'm goin' to Paris!" - P.W.
The world loves diversity... as long as it's pretty, makes them look smart and doesn't put them out in any way.
There's the road, and the road less traveled, and then there's MY road.