The pattern instructs you to use a yard stick and chalk, but really…really, if you don’t already have a rotary cutter it is time. This is Shawn kneeling on the dining room table that was formerly known as mine. On our last trip to the fabric store together Shawn bought a set.
If you are worried about the size here are a few things to consider. The pattern is for an open bag. If you want to be able to close it I would add about 4 inches. I left the external fabric the same length, but added 4 inches to the lining that peaks out of the top and draw strings shut. Also, my yoga mat is double thick, but had no problem fitting in, so I wouldn’t add anything to the width.