If you bought the kit, you must first remove the silver rack from the bike, then the large black triangular plate from the rack. The kit pieces now install on the black triangular plate to create a latch system to fasten it to where the pillion seat was.
If your go back and forth frequently, it is easier to order a second large black plate and modify it. Rather than remove the one from the silver rack and have to re-attach it to go back and forth. Of course that is more $$.
Here's a picture of the mount plate in place: