So many of us know that's not fun at all because we've done it ourselves - some of us more than once. Here's the way I do it now, just to be sure:
Of course, measure the top and cut the backing at least, repeat: AT LEAST 3" larger each way.
After pressing the backing piece, fold top to bottom and side to side, i.e., carefully in 4ths ... make sure edges are lined up.
On the right side of the backing, place a pin right where the center point is.
Place another pin in the fold at the outside edge (which is the outside center.
Open up the backing and lay on a flat surface.
Now, do the same with the batting. Lay the batting on the backing, matching those pins. Remove the center pin from the batting. (You'll still be able to feel the center pin on the backing piece.
Now, do the same with the top. Repeat matching the center and edge pins.
You now have a perfect register and if you're short, it's gonna show before you get any more work into it. Of course, you'll still kick yourself if it is short, but at least you've spared yourself a lot more work.
Hope this helps.