First look into the light source and really try to understand it and/or set yourself a plan for it, where it's coming from, how far away it is, so you get a good grasp on how sharp and diffused the cast shadows would be. There's Plenty of good lighting references for real life portraits on sites like Pinterest, so you can get a bit more of an understanding of how light interacts with the face under different light intensity, light direction etc.. Another thing is I'd go over with a small brush, pushing darker colours across the drawing, especially in areas where light isn't hitting and areas you want to emphasize form and indicate more three dimensionality. An example would be for the eyes and hair. Another thing to look into is how to blend shadows naturally, especially here where the light seems a bit further away which would mean the shadows aren't as sharp on the face, really think to yourself "where is the light really hitting, how far away is it and so on." I didn't do a full paint over but i hope this helps! | Artwod Feedback