Scenery installation and .afcad files.

I have always wondered about these and because I approach fs9 with a renewed enthusiam after a long layoff, my purpose is to start tweaking and replacing as much of the default scenery as I am able. Bottom line: I don't have any clue where to start (except the Avsim fs9 file library) and I would like to be pointed in the right direction by a kind soul/s, here in the forums. I need to know everything from how to install .afcad files or whatever you do with them and well....everything. Any help will be appreciated.Jon