This is how I paint my drawing.
Theory: Any figures consist of simple geometric figures. All you have to know is how to image your drawing geometrically, plus how to shade Cube, Cylinder and Sphere.
I suggest start from monochrome. Here is one of my wip painting https://p.twimg.com/AqFjUMRCIAEqzkp.png:large
Firstly, decompose a human form into 5 cylinders and 1 sphere ―― a trunk, 4 limbs and a head. Then shade each simple solids roughly.
Secondly, go into details. for example, an arm consists of 7 cylinders and 1 cube ―― 5 fingers, an upper arm, a lower arm and a palm. Then shade each solids carefully. Repeat this on every limbs, trunk, e.t.c.
Finally, go into more details. Repeat this until you satisfy the result.
Hope this helps