ok how do i shade stuff?

[clever rover]

in order to start drawing in the shadows, you want to identify the light source, so you can determine where the shadows will cast according to the curves. in the example image you added, you can see that the light source comes from above, since only the top parts of his facial curves are lit

essentially like this

the way I do it is that I do largely the same technique as you, with the multiply overlay, and then I determine the light source and erase the areas where the light would hit the curves