Same here, I've been mixing and matching themes to get what i want and I've got a pretty good understanding of where most of the files are and how they work with winterboard. A good place to start would be to get that stock ios4 theme that is on the first page of the forum right now, that way at least you know where all the files are located and what they change. What I would like would be some guide on the photoshop end of things.
i would say download some templates first, get to kno the files , there sizes and names and how to install them manually , after that, you should be ready to give it a go and you would be good!