try level shadows
on a new layers, also try to copy a part of your drawing on this new layer, and flip verticaly also try to distort or modify respecting the colors darking , lighting or also angle of the light on you drwaing it appears to come from the back so your shadow will come form the back to the front around the leg or arm.
 i think also this way because i am a oil painter so you need to look at the parts and klip move turn or distort and respect the colors but make them darker or lighter depends of light effect.
regards Oneric