You will be able to download the weapon UV Map that is located in the skin section of the armory menu. Essentially the UV map is the texture wrap that goes on the weapon model. So in order to make a skin you will need to edit the UV Map. This can be done in many differnt types of software. Blender is a good one but a bit advanced. I would suggest using Pixlr: pixlr.com as it is easy to figure out. Please take a look at this though as it is important or you won't be able to publish your skin: https://warmerise.com/forums/topic/1300/verification-pixels