However, I looked at it today and I cannot figure out how to attach it, at all.
I tried attaching it to the rings my saddle has for a flank cinch, but the crupper isn't that long... Help/ideas? I guess I could tie some baling twine though the flank cinch rings in a loop long enough to attach the crupper on each side...
Pictures:
The saddle (I don't have a better picture of the very back, I can get one if it's necessary...):

The crupper:
Leather Single/Double Strap Adjustable Saddle Crupper on eBay!
I had thought I had it all figured out. Apparently not! Haha