It's kinda pointless to start without a template really, but I've never seen them post any templates to help people create skins for vehicles or uniforms, though these undoubtedly exist. The only templates I've seen them post are for mission briefings. I guess if it is just small changes to existing textures you can do it, but a complete overhaul or new texture is pretty difficult. The texture map above is a token gesture, there are only a couple such maps in the game files for any of the games. In summary, there are no tutorials, starting points, help, templates etc, it's kind of amazing the modders who did make new textures got anywhere at all (maybe they did get a bit of help? - who knows? ).
You might try asking a modder who makes skins for the game you are interested in, they may have something. Perhaps ezjax for WW2 uniforms, aris/fuser for WW2 vehicles but some of them are less active these days and may have deleted whatever resources they had.