I'm not a big hair expert. I am in a lot of trouble too, but I try to help you. Before working on the light and shadows of the hair, try to define the locks and the points in which to insert them. Then you start to set the light points according to your needs, dividing them into the various gradations. (Here I only worked on a small section. I didn't want to modify your drawing too much). I repeat I am not an expert. Other users probably have more knowledge and more effective advice.