Wash, dry, and press the fabric.
Hold the fabric by its selvage between your fingers and thumbs.
Hold the fabric again as in the first picture and move the fabric back and forth between your fingers and thumbs until you get the fold of the fabric hanging straight on both sides. Once you have it hanging straight, lay it on the table. Using your ruler, line up your ruler with the fold edge or the selvage edge, cut off the long edge of the fabric to have a straight cut (In this example, I cut on both sides for the pillowcase hem). Sometimes you might have to cut off a lot, and sometimes you don’t have to cut off very much at all.
Obviously, scraps or small pieces of fabric that come in kits can’t be straightened by this method and rarely need it anyway. I look at the threads and try to line them up with my ruler and get it as close as I can. For the big pieces of yardage, I just do my arms length at a time and straighten as I go.
So, why go to all this? Well, we want our quilts to hang straight, we want our pillowcases to not twist up, we want our tablecloths and napkins to lay nice and flat and then, when it’s time to put away, fold up nicely. Straightening the fabric helps all that.