Start by reading this article that the DFW group wrote up:
http://www.aquatic-plants.org/articl...ges/index.html
That should answer many of your initial questions. In order to rely on the substrate producing the CO2 you need, it will need organic matter in it. You can get this by taking some soil from your yard/garden and making a 1" or so underlayer in the tank and then top it with your sand/rock substrate.
-Dave