I have not been able to figure out how, a step by-step process would probably be best.
Go to Edit Mode by pressing [Tab]. Press U and then click "Smart UV Project". You can adjust some of the numbers to change how the texture is generated. You can also change the positon of the sides on the UV Map. To see textures, go to Textured mode
Afterwards, draw on the sides. You can do a test of which is going to draw on what side by drawing numbers on the UV Map or draw marks in Blender(I think there is a way to do that, but I don't know how). When you are done making it, save it to your computer and open Blender again and import the image.
After you have done that, you will see the texture you have done in the Textured rendering.