First, I'm going back over my first four Steam Skins - Luna, Chrysalis, Celestia, and Octavia - and updating them. I'll tweak them by adding colors, perhaps changing images, and overall making them look better. If you have a problem with one of those skins, go ahead and either note me or leave a comment on the Skin: I'll be sure to take a look! Anyway...
Some people have asked me to make custom Steam Skins for them. As you can see, I'm dabbling in Vector Artwork now, so I'll have something to do between skin requests. For the most part, the skins I make are on request, and I have no real list. If I have nothing else to do, I'll gladly just start making more Steam Skins.
The point is that I'd prefer if people knew what I'm looking for before just asking me to make them a Steam Skin. If you want one that's the same as the ones I've already made, except the Mane6 Fighting is Magic skin, here is what I use...
2 vectors, a wallpaper, cutie mark/related image, image related to the character, palette, font.
All I really need to make a Steam Skin is a palette if you just want the colors to be different. Anything in between is also good, and even beyond would work. Just remember that, the more images I put on there, the longer they'll take to load when you open the Steam UI.
If you have any questions, feel free to ask in the comments! If you have everything I need to make a skin and want one, go ahead and note me, and we'll go over the details.
NOTE - If you're interested in what art I use specifically in any skin, take a look at my "Used In Skins" category in favorites. Everything I use in a skin goes there after the skin is published.