Portrait sketch composition: after determining three courts and five eyes? How to find light and shade

First, determine the direction of light. The whole sketch head is composed of gray and black, and the most prominent part should be white. The distance from zygomatic bone to masseter muscle and then to the thickness of chin can be understood as a line of intersection between light and shade. However, it should be noted that the two lines need to transition a plane, and then the lines should be arranged from the shape of "intersection of light and dark" to the dark part, and the lines should be arranged upward from the shape projected from the neck. It should be noted that the sides are heavy and the middle is light, giving people a sense of lines.