Sketch a circle where you want the light source to be. You don’t have to keep it in the final drawing, but it will make it easier to determine where shadows would be. Consider sketching a few light lines where the shadows will be, so you won’t have to think about it later on.
Work in small circles to create an even tone.
Keep in mind that, for example, hair isn’t very reflective, while eyes are. Look at reference pictures to see how reflective a surface is.
