To start skinning a plane do this: Download and install the terrain editor.
Make a new terrain, choose a shape (airplane or building or whatever) from the objects list then push above where it says "save texture". Now, in you terrain editor folder under the name of the new map you made, will be a new folder with all the BMP files files for that plane, change the BMP files to your liking (don't forget to MIRRIOR all images then Rortate then 180 degrees). Save the files you've changed then copy the files you've changed and make a new folder in your terrain folder (the new terrain folder that was created when you created the terrain, name of folder is same as name of the terrain). Name the folder "texsrc" and copy all changed imagies into that folder. It is now done.
A skin can ONLY be created for a terrain, and only one skin per plane. Thus you can't create your own skin and put it in the MA but the skins created will only be created for a special terrain.