Once you have the entire quilt top put together, take it to the ironing board and press all the seams in the same direction. Don't stop to press in between. It takes extra time.
When I had the top put together and the seams pressed I took it to the cutting mat and measured it both ways. Because I suffer from "OMG, I can't remember!" I wrote down the numbers on my little black board. Then I cut the backing. I found this was actually a better way to do it than pre-cutting the backing.
I had a yard of fabric, or at least I thought I did, but when I straightened it up, I discovered I was short
1 1/2" on both ends. I trimmed the front to make them fit.
On a few of the "over" quilts I made I decided to round off the corners. I took a coaster and marked the top with my favourite marking pen. When I sewed the curve, I let the edge of the sewing foot follow the line.