Its like reading a books, took me so long to draw it. Go to Google and type in muscle anatomy, understand what lays beneath your skin, next use yourself as the model and experiment with different light angles and take pictures. Your body is the best tool, and finally for facial expression do not be afraid to use the mirror to sketch. When you understand the muscles you understand shading and depth. If your unable to draw perfection, DO NOT get angry , the way you draw is unique, the right person will come along and want you to commission. Thanks for the question, all the best