Longarmers: Want to be sure you have a straight edge along the top of the quilt, but don’t have a horizontal channel lock function on your machine? Use a small C-clamp like this on the vertical track of your machine. It will allow you to baste a stitch a long straight line on your batting for you to lay your quilt against. With one straight edge, you can square the others accordingly.
Of course, you remove the clamp before quilting.
Note: many great longarm machines do not have a channel lock feature on them. The channel lock is made with magnets, which are heavy. They add a lot of weight to your quilting machine, which may not be ideal for dense, custom quilting.
Yes, there are fancy laser lights that you can get, but this is cheap, easy, and you know you already have what you need in your garage.