[Yasny]

First of all make sure you have the channels window open (click windows, then show channels) There should be a channel called alpha. (or alpha 1 I'm not sure. something with alpha in it anyway) You should now have a black and white image, showing a silhuette of the normal image. White is visible, black is invisible. Greytones are semitransparent. (I've never edited a skin myself, only scopes and compasses, but I suppose skins are pretty similar.)

As far as I understand, what you need to do is make the part you want to cut out black. A good way of doing that might be to first view the skin in normal mode. Then mark up the part you wan't to cut out. Switch to viewing the alpha channel and use the paint bucket to make the selection black.

How this will look in the game, I have no idea. But I think it's worth giving it a go, who knows, it just might work.