Before you get very far I should go through a few things in an effort to help you avoid a few pitfalls:
Any changes you make to any skin are for your own personal use only and it is not to be distributed in any way.
Regarding the skins that ship with WB - As far as I know the only source for the original skins is with the installation of WB, and those skins will get over-written when you do an installation of WB or install a WB update. To deal with that you can either:
Back up the skin before you edit it to retain the original, and backup your edited version so you can replace the version supplied with the update every time
Use SkinStudio to create a copy of the skin under a new skin name and edit that copy instead of the original. There's a picture of how to do that in Reply #7 here: