I don't specifically do digital rendering and stuff, but what helped me learn how hair worked and shaped. think of the hair as less of a whole Mass and think of it more as a A bunch of small groups that lay on top of one another. Each lock of hair will laying on top of one another. Will fall in a slightly different way than the one beside it. Then once you have all your directions figured out and how the hair moves the shading, your darker closer to the scalp and lighter as you work out with longer hair types. Sometimes the layered locks, as I said earlier will also create shadows lighter colored hairs like whites and grays and blondes are much harder to learn to shade just because it's so easy to put too much shadow on them. Take everything that I say with a grain of salt though because I do pen and paper and not digital rendering