The nylon back cinch you posted also has the billets with it, so the leather piece hanging on your saddle will be replaced.
After undoing the buckles, you just feed the end through the ring on the saddle and back through the loop on the back cinch. Those pieces will stay on the saddle like the leathers billets that are already on your saddle.
Edit: And Phly beat me to saying the billets are already there. And I agree with the getting the leather cinch. Not for aesthetics, but the nylon one I had rubbed the horse really bad.