If you are wanting to know how to create a skin from the begining (design) to completion (coding/launching), one method is to look at a default skin. Get into the code and figure what elements are graphic and what elements are code based (i.e. a button could be a graphic where a line could be code). Using that information you should be able to see how it is designed/sliced to complete the skin.

Then crank up PhotoShop or your fav. design tool and go at it.

Good luck!