I followed this very well done video by Jerry Bates. Looks like there are a lot of them to watch, so that will be fun to go through. He talks about the TwentyTen theme here, but it works just fine for this theme…and I am sure it is pretty standard. He uses a free online pattern generator called BgPatterns where you pick various images, colors, transparencies, size, canvas texture. I was trying to match the sky in the balloon header that I have up right now. After you have the pattern how you like it, save it to your computer. Use the Dashboard to adjust the Appearance and the Background. Upload your background there.
Simple and easy to understand. I think his video series will be quite helpful.