It may be easier to buy than to make your own (if you are talking about wrap skins) although if you have something fantastic in mind, well, you can make your own template by measuring your mod, etc. What materials are you planning to use? Do you want an "all around" kind of wrap, or just to be able to make parts of your mod look different, using stickers? Either way, you'll need the dimensions of the mod, as well as probably some practice in order to make them look great. But as far as actual templates? I guess you could ask the manufacturers, too, although they may or may not be willing to give out than information (or really probably, vendors who make skins as they've done the initial work for their own business, so to speak).
Good luck, however.
Anna