After I quilt on my longarm I use my 12" square ruler to square the corners before adding the binding. I mark the areas off the square with pencil, sew the binding to the pencil marks and trim the excess before folding and sewing down the binding on the second side. If you trim before adding binding you may have trouble with loosened batting getting in your way. I must have learned this from someplace but really don't remember from where but it does work for me.