In the process that Jeanne S describes, when you get to the point of cutting your 2 1/2" strip, when you line up the 2 1/2" line on the ruler with the edge of your fabric (make sure you include the line in your measurement), if the bottom fold of the fabric doesn't line up with a horizontal line on the ruler, you know your fabric isn't folded properly, and you will get a bend. Unfold and try again. When you have a side edge of fabric that lines up with a line on the ruler and a bottom fold of fabric that lines up with a line on your ruler, you know your fabric is straight, and your strips should come out straight. I often have to refold after I've cut several strips.