A while back when looking at the coloring stuff available at Wal-Mart, I discovered a couple kits designed for kids regarding shading with colored pencils and markers.  They were $5 each, IIRC, so I picked up both and figured if they were for kids it would be technique I could manage

This is the colored pencil one that I did when I first got the kits.   They come with 15 pages to color and colored pencils labeled in each "set."  There is a light, medium and dark for each color set.

Since these are made for children, they are not exactly sophisticated designs, but you can get the idea.

So the idea is that the light is from the corner up there in the left and you shade based on that.  You can see the letters since I don't press down all that hard when coloring. 

When light shines on something it has a light, medium and dark side.  It will also cast a shadow.  In the pages they give you, the areas are very clearly delineated, so blending is not really possible.  The idea is easy to understand with these pages, but they don't look as good as it would with blending and the areas not so obviously marked out.

The shading pretty much works like you would expect.  Things the light hits directly are the lightest and the areas that are opposite the light sides are dark sides.  The area between those two is the middle.  Then there is a separate shadow that is cast on the background.

You can see on the cattails that there is no dark side because the light all of area to some degree.

I was going to make separate posts for colored pencils and markers, but after some thought since the concept is the same with both that isn't really necessary.   Here is the marker page I did to test it out.

On this page you can see there is not only a dark side on the horse, but there are also several areas that have shadows.  The base of the tail, the top of the back leg and the base of the mane.  Then the horse casts a shadow on the grass, as well.