All right, sorry if my hand righting was terribul, dont blame me it is late -.-
Step 1: Add two layers one for shading and the other for color. Make sure the shading layer is above the color layer.
Then change the shading layer to 'multiply.'
Step 2: Use the pen tool and take all different shades of greys and add them to where you think it should be lighter and darker c:
Step 3: Then use the smudge tool, and then smudge it a bit. Then use the bleder tool and blend it together a bit to look more realistic xP
I hope that helped Jukebox x3