Shading is creating shadows, or highlight parts of the texture.
For shadows, just imagine a light source falling from the top of the texture to the bottom, and you will see where shadows should be placed.
If you are using Photoshop it works like this:
1. Find spot for the shadow
2. Draw small black line on that place
3.Use Gaussian Blur on it
4. Maybe lower the opacity until you like the result.
Its pretty much the same for highlights, but you use a white color, and you put it, where the texture should "shine" a little.