I, too, have used plain old scissors for a long time. The only thing I would add is that when you make the cut, you should try to make it curved rather than cutting straight back (parallel to the edges of the film) and then making another straight cut angled to the edge of the film. I used to do it this way and had a couple of leaders tear off in the camera. The tear started right at the angle of the cut, so now I try for a nice curve just like the factory ones.

Bruce