question

Solution
You upload skins from the marketplace. As for making them that's a bit more complicated. It needs to be in some sort of hosting service like imgur as a texture for the gun
In order to make a skin you can either run the /market command ingame or go to the security sector and look at the skins. Once you have done that, you should see a little button in the top right of the GUI, in which you can then go and create skins.