Well the very very basic run-down is that you need to save the default skin, edit it, and then export/saveas an 8-bit bitmap. How you get the desired results you want is where everything changes.
As an aside the old skins no longer work on the new models, as they have been re-mapped (different parts of the bitmap are used on different parts of the plane now, so using the old skin things show up all messed up).