The best way is to open an existing skin and mess around with the graphics and settings. Choose a skin that's somewhat like what you would like to accomplish. Probably be best to make a copy of any skin you choose to alter, though.
Also, like gmc2 stated, it would be helpful for those of us who might be able to help you, if we could see a picture of what you're working on...
**disclaimer** it's never ok to just make a few modifications of someone else's skin and upload it as your own. 