Once you have a UV mapped 3D model, you work in an image to build the texture map.

Do you have the "regular" or the "extended" edition of Photoshop?

If you have the extended edition, you can create a 3D layer from a Uv-mapped model, and start painting right in the texture map, and you can see the results very quickly.

There are lots of ways to go about it, but a common method is to use pattern stamp to clone some photographic source to your texture map.