I have found that some people really frown on bringing the backing to the front for binding. Quilts of Valor and Linus Quilts will not accept them. But we can do as we please. I have made dozens that way. I always sewed them down with a decorative stitch.
But when I turn the corners as Charlee suggested, I always make snub nosed corners. I take that back. One time I made a perfect corner like that. I should have taken a picture! I make them anyway. Who cares.
But my sister showed me another way that makes really square corners every time. When you have folded the back over, turned it under and stitched to a corner, stop about a seam's width from the corner, keeping the needle down. The back/binding fabric is folded to the end. Turn the bottom corner up on the diagonal so the fold is even with the edge of the quilt. Now turn the next side of the back/binding up and turn the edge under to sew down the next side.
This works best if the back is trimmed to the exact width needed, and you may want to trim a little excess out of that corner. It gets kind of bunchy. Make one, and you will be able to tell where to trim.
Good luck on your back binding.