all you need to do is find repeating textures (images that repeat like wallpaper). There
are many out there. Here's a good website:
I found this site by doing a google search with this:
free + tiling + texture
Then, once you have found your nice images, scale them to 256x256 with your favorite
paint program. Then save them as .jpg and put them into the skin zip file. You need
to have the same paths and filenames as the original images in the zip file. Probably
the easiest way to do this is to extract the zip archive, replace the images, and then
zip it up again.
Yes, most games do use repeating textures. However, I want to avoid using textures
from copyrighted games. So, please find textures that are free to use.