I usually sew a basting stitch all the way around the very edge of the quilt to keep the border edges in place. That makes it easier for me to square it up. I use a large 15" square ruler and the long 6x24" ruler to align along the edges (starting in a corner). I use a large table and push the cutting board along the side I am trimming. Usually there is a specific line in the quilt pattern (or border) that I use to align my template. Most of the time I do a dry run with the template all the way around the quilt to make sure nothing is too wonky. Not sure if this helps. Good luck.
Oops, didn't read that you already figured it out. Good for you!