It's fairly simple, but requires practice, here is a few tips incase you wanna use them, they helped me..
1. Establish your light source, decide where the lights coming from, and then shade accordingly.
2. Practice copy. If you draw a really great picture but your afriad to shade it because you don't want to mess it up, make a photocopy of the drawing and shade the photocopy and if it comes out nicely, just reshade on the actual drawing. (Good thing is also that if you mess up and erase, it doesn't erase the lines of the drawing, just the shading, this is a great practice technique.)
p.s. to anyone who cares - I made a gallery for my work and I might put other people's work there too -
click here