Simple but hard to explain, very light base layer gry color set to multiply because its a consistent color, you can substitute other colors for shading if you want. With another layer under that also set to multiply to do "ambient occlusion: around the details and darker spots to give more depth, another layer of blue set on top set to overlay, clipping to the base shade layer to give dark spots darker, then an overlay layer of the same base shad color to lighten things up a bit. Highlights are 2 different colors of dark grey, one lighter and one darker, set the layer to glow dodge and use the darker grey for highlights and the lighter grey for more shiny parts and fleshy bits. Then make another layer of a darker grey and set that to glow and go over the character to give more focus on where the light source is hitting.