I share your opinion on the documentation in places, it could be more tutorial based I feel.
If you have some experience with xml already, perhaps you should take a look at my skin Beaver and Steve -
http://www.msgpluslive.net/skins/view/8-Beaver-and-Steve/
I use shaping in there using a masked image.
Basically you have to use the 'add' function rather than subtract
and you should get your shape applied