I bought a bigger cutting mat (30" by 60") in the hopes it would make it easier to square up the edges of a quilt top. I tried it out - it made it a bit easier, but it's still a pain and not accurate.
I squared the top and one side and then worked off each edge to square the next side. It didn't work. The top is longer on one side than the other. Good grief. I'll figure this out later.
In the meantime, I got to work on the backing anyway. I found a big chunk of fabric, a two yard piece of a Kaffe Fassett design with flowers that had been fussy-cut and had holes here and there. I cut out pieces between the holes and this is what I ended up with: