I have a problem being on my knees - I can still get up off the floor but it is NOT a pretty sight!
Anyway. I have a fairly large table and line up the backing on that. Then I weight the sides of the backing that hang over the edges.
I fold over the edge of the backing and pin it - and then put a yardstick or curtain rod or PVC pipe into the "pocket" that I made and that helps keep the backing from rumpling or wrinkling when I put the batting/wadding and top over it.
It just adds enough tension to keep it from wadding up on the back. That happened to me a couple of times and I did not enjoy the unpinning and repinning process. It was hard on the fingers.
I also tape the ends that are on the table to help keep it from shifting.