I put the selvages together and if there is a wobble at the bottom I scoot over the back or front selvage until it all hangs straight. Then I place it on my cutting board and very carefully fold it in half . I then put my 8 1/2 ruler along the fold and my 6 x 12 ruler along the raw edge. When they butt up perfectly I remove the 8 1/2 ruler and make my cut. I try to make as skinny a cut as possible but sometimes the fabric is so out of wack I have to cut of more than I would like.