So here's what I do. I trim down the top two layers of my quilt sandwich to the same size, and leave the backing about an inch larger. It's no problem to fold the raw edge in and fold the backing over the front along the sides then; but the corners still take some maneuvering.
The first step is to clip the corner. That point will just get in the way and make things bulky. Snip it off.
Then you can fold over the first side using binding clips to secure it. (Or pins -- pins work too.)And then fold in the other side. Just in case you're not certain what I mean... we're bringing the raw edge of the back to the raw edge of the front by folding it. Like this:Then folding it again. This time that fold line is the spot where the raw edges met; the backing will come over the front of the quilt and neatly hide all the raw edges.
At that corner I use the tip of my pin to make sure everything gets tucked in smoothly and neatly. And then pin it firmly in place.