I have a Wunderlich bag for my R1150r with the same fixing system. From memory I would say you are missing the webbing strap and buckles from the part on the bottom left, the strap goes around the head stock and the buckles clip onto the short pieces of strap. The bag clips to the other end of those short straps, the bag connection is hidden under the elastic.
The part on the bottom right is attached to the tank with velcro, you will need to stick a piece of heavy duty velcro to the tail end of the tank below the front of the seat. The small strap and buckle then attaches to the bag, once all three are connected you can pull the straps for a secure fit.
I was a bit wary of the fixing method when I first got my bag but during three European trips and countless UK trips I have never had a problem with the bag coming loose.
At fuel stops I undo the front connectors, the ones under the elastic and fold the bag back onto the seat.
Hope this makes sense, if you need more info I will try and take some pics later in the week of how mine fits.