I don't have my buttplate finished yet, but my understanding is that the there is a bolt that goes through on each side, the same style bolt that attaches the collar and backplate together (looks like a keyboard key).
I used the bolts as well, but on one side I just attached the bolt but it doesn't go through, I have velcro there so it opens up without undoing the bolt. I also attached velcro to the cod and butt and the jumpsuit to keep it all in place.
The bolts that connect the collar and back armor (I call them "collar cubes") are the same thing used on the butt/cod connection. I personally used snaps with a reinforcement of industrial velcro.
I used velcro on the buttplate to attach to the cod, I have the little bolts also but they are just for looks. As for the backplate I used the bolts on that one I just put a screw threw the armor into the bolts on both sides. To take it off you just unscrew the 2 bolts on one side.