1. add a new layer, shove it below your line layer, but keep it above your color layer
2. make the blending mode of your shading layer either:
-Difference
-Exclusion
-Subtract
These all are great if you want different types of lighting(indoor/outdoor)!
3. Choose a colour. I usually use blue for shading, but you can choose whatever you want.
4. If your art were 3D, find the points of the art, and go from there, shading in one direction(where is your light source? go opposite from your source of light.)