I square up one end. Mark 7 inches in along the length. Then I use a LONG straight edge/yard stick I bought at Lowes & draw a line connecting the marks. This (see link) one is 6 foot long & not too expensive. I have another than is 4 foot long. They have a hole drilled in the end for storage. I store them via a screw on the sewing room wall behind the door. This keeps it from getting bent. This is the 6 foot one I bought:
http://www.lowes.com/ProductDisplay?...llow&cId=PDIO1