check under the "Aces High Tour of Duty" column. You can only do skins in the new AH2 , and those have to be "historic" and accepted by HTC before they "may" be added. ( altho, you can add new skins to new terrains as you build them I believe in AH1 to be used in H2H or senerios)
I don't think HT has come out with a set of guide-lines as to how, or when he will bring in new skins. He has however posted rules about what type, and how to submit skins there.
As for "how" you do it, any decent art program that uses "layers" will get you going in the right dirrection. Some of these guys make it look easy, but it isn't
