I draw one line in the middle somewhere, then take a 6 x 24" ruler and draw lines 6" on either side. Then I draw the lines at 5, 4, 3... well, like that. Helps keep the 6"ers stay parallel to the first line. If it's a big quilt, I keep adding 6"ers then in-betweens or make marks and lines at 12" and so on.