There's some demand for niche accessory boards for the RPi. Relatively easy to design, too - get a basic copy of Eagle or KiCAD. I don't know if any single tut

There's some demand for niche accessory boards for the RPi. Relatively easy to design, too - get a basic copy of Eagle or KiCAD. I don't know if any single tutorial is going to give you what you want. I'd just dive right in, figure out what you want to do with your board, select some parts at Digikey, and start designing. Google questions as they come up.