Originally Posted by Scissor Queen
Measure off about 80 inches along the selvedge and snip it and tear it. Yes you'll get strings and fraying but it will be straight and on grain. Do the same for the other direction and give yourself about 6 inches extra.
This is the method I use when using a large piece of backing fabric. It's easy, removes the stress of having to cut a long, straight edge and has always worked for me.