Does your explanation have to exactly describe FTM's data model? Here's a (woefully-outdated) description of The Master Genealogist's data model. If it's only the sources/citations/media & "facts" entities you're describing, how about using the Gentech data model as a jumping-off point?
Re-reading your question, I think I'm probably off-base here. It sounds like you're trying to help a FTM user to locate & fill in the appropriate FTM fields/attributes to document their sources, not trying to help them understand how a genealogy app works "under the hood".