Ive been wondering too, how to do it. Got a rag installed, just to realize, that the backfork istnt made for big loads. But as there is sooo much space under the bike it turned out to be the easiest thing in the world. Attach anything underneath! Click and strap to the seatpost. All load is suspended, welll centered; so simple!
Dont worry about loose straps. Just wipe them together in the strap around the bag, so nothing hangs down in the road.