I have a 5 and 6" 24" rulers - I butt them together and tape them - love duck tape - I make sure my selvages are together. I put straight pins on the left to keep my selvages together - prevent them from slipping. I have 2 mats and make a continues cut.
I also - probably not quilter correct - but after I have folded my fabric, done my straighting cut, will put pins about 12-14" below my ruler -I find that this keeps my fabric straighter - move pins as I cut - can make more cuts before straightening my cut line