There are several ways to do it...

The easiest is to draw the assign a texture over the top of it (composed in Photoshop or the like)..

If you want better detail, you can use detail mapping I think it is called, or a multitexture shader, where the textures are blended according to a blend map (each colour channel determines the blend percentage of that texture)..

The latter gives the best visually, but it harder to get working

Adoado