First of all, to create one you need software that can do it.
I suggest free ones like Paint or Paint.NET if you are a beginner and GIMP if you want something a little bit advanced.
You can goto the textures part of the shop and look at the mapping of the textures by looking at the templates next to the name of the texture you want to make.
To equip it after you have made it, you will need to buy each individual body parts.
The available sizes are:
128x128 - 2k
256x256 - I'm not sure, I think its 5-15k
512x512 - 20k
The higher the number of pixels, the higher the quality.
Hope this helped.