Here is something some might find useful.
If you have trouble getting values, highs and depths right on drawings you can practice this. Draw a face from reference and draw lines into it. The lines should be like a 3D pattern mesh and emphazise the human form.
My mesh is not perfect or anything, but I really liked this exercise. I guess one can do it on photos too. Like taking moms beauty journal and go liney on it. xD