I used to sharpen snuffers by laying down a large file and pushing the head on it. Now I hold the head and file across the top.
I mark the edges, and file each side to true up the angle. When the marks are gone, I file a side until I raise a bur that I can feel and see. I then go to another side and repeat for all three sides.
I then lighten up my strokes on each side, maybe 10 strokes of medium pressure, to each side.
When I can no longer feel a bur I do a series of 10, 8, 6, 4, 2, 1 very light strokes on each side, rotating each set.
I finish by pulling lightly 2-3 times on a steel and test by shaving hair off my arm.
Maybe there is a way to get them sharper, but these seem pretty dang sharp to me!