Fringes in leather garments should be only as wide as the leather is thick. In other words, the fringes will be square.
To cut them, I place the leather flap that is to become fringe on a kitchen cutting board - the soft self healing plastic kind with a kind of embossed textured surface. This grips the buckskin, and holds it from sliding around while I cut. I use a short bladed knife with a curved edge, that is razor sharp, and a steel straight edge. Once I have made the first cut, I simply slide the straight edge down the seam about 1/8" and press it down. Then I use the tip of the blade to cut right to the seam, and slide it down the straight edge until I run off the end. From time to time, the end of the cut leaves the fringe with a fat end, and I just trim them off.
Fringes along the edges of garments should be more like hair than fringe. They don't have to be long to be attractive and effective. Brain tanned buckskin makes the nicest fringe, by a long shot.