This is how I was taught. Iron your fabric, selvedges together, a nice sharp crease on fabric fold. Place on cutting board, place a right triangle along the crease, it's bottom edge lined up straight with the fabric fold. The other straight (verticle) edge along the left open side of fabric, (moving the triangle in enough so you have enough fabric to cut off along its edge.) Take your ruler and place and hold it against the left straight side of the fabric. Carefully remove the triangle. Cut along the rulers edge, holding it so it does not slip. Remove fabric strip. You now should have a straight and even edge. Now you can cut straight strips.