Don't you hate it when a pattern calls for a certain size ruler and you don't have it. These quilting rulers are not cheap, so you don't want to go out and buy one every time you need a different size.
I took a quilting class these past 2 weeks at a wonderful shop called Whistle Stop Quilt Shop and the instructor Connie showed us this neat trick.
Just use painters tape to tape off the size you need. Cut 2 edges, then rotate and cut the other 2 edges. Done and done!