It takes some getting used to, but is a breeze after you've been doing it a while.
The backplate is a very precise fit. I generally just make sure the entire lower is pushed forward and up as far as it can possibly be, and start by putting the bottom edge of the backplate just over that part of the lower that extends back, then sort of lifting up/tilting forward...side to side twist as well if need be. Just look at all sides as you try pushing it on, as everything pretty much needs to be exact.
It's amusing how seemingly difficult reassembly can be until you get the hang of it, or something finally goes into place. It's like, F***K, now why was that so difficult?
I've certainly had my fair share of trials early on with the locking piece/bolt assembly, as well as getting the stock back on.
Just keep working at it, and don't force anything.