Be sure your ruler bottom (nearest you when cutting) and ruler top (furthest from you when cutting) ar on the same line for cutting.
As you cut, cut about one hand length, stop, walk fingers up ruler checking ruler line, cut again, and keep doing this until you are done.
As you begin cutting, you come into the ruler edge at an angle and then straighten your blade as you cut. One should not be cutting into the ruler plastic as it nicks the blade. The sharper the blade, the sharper your cuts.