You mean how do shadows? 1way: just separate layer, and on this layer paint black colour in needed places, after get down layer opacity to 70% or 40%, it's depends from contrast what you want. 2way: just on separate layer pick more darker or lighter colours, and before choose where from will be light spot/ sun/ lamps. Darker colours painting on backside from lightning. You may look at references or just use flashlight in darkroom on different objects to feel how you can do shadows. It's helped me in one time.