Well...come on, it's a seat bag. What's to learn about it? You look at what you want to carry, and you go to a bike shop and buy a bag big enough to hold it. I have maybe half a dozen of them, from just-big-enough-for-a-patch-kit to this:
http://www.blayleys.com/trips/2002/4...3-1308_IMG.JPG
Over the course of a summer, I use them all many times.