First off, in AH1 you can't do skins to use on existing maps. You can only do skins on new maps that you make yourself, and as such are only visible offline and in H2H play on those maps (unless HTC uses it in the MA). AH2 supposedly will change this but so far this feature doesn't exist. So for now, you're stuck with AH1 rules.
In AH1, first you need to get the terrain editor to make a map in which to incorporate your new skins. Then you need to make a small map, such as the tutorials give you. Then you can make skins to your heart's content.
To make a skin, open the terrain editor, select the plane you want, and hit the SAVE TEXTURE button. This exports the texture. Then use some good graphics program like Paint Shop or Photoshop to make a new skin, using the stock skin as a guide for where to put stuff. Once you've got a new skin made, put it in the \AH Terrain Editor\My Terrain\texsrc fold for your new map. Open the editor and build the map again, which will put your skin in it. Then move the My Terrain.res file to your AH\terrains folder, go offline, load that terrain, and your new skin will be there.
