Salvage edges are woven tighter than the fabric yardage, often causing them to not be straight/ even. Start with cutting off the salvages so you have straight edges. Fold your fabric in half once making sure your fold is straight. Placing the fold on a line of the cutting mat remember the fold is your straight edge, don't fuss with the raw edges. The straight fold edge is the part that is important. Place your 45* (degree) line of your ruler on the fold and cut your strips.