I prefer to perform this step using my 2” x 72” knifemaking belt grinder. For the initial rough sanding I often use an 80 grit belt, bringing the edges of the leather down until all three layers are even with each other.
I then will use a 220 grit belt to lightly smooth the edge and chamfer the corners for a nice professional appearance. Once again, it will help prevent burning the leather and help to achieve a smoother job, if you first dampen the edge with water.
If you don’t have a belt grinder available for this step, you may well be able to even up your edge using a wood rasp and then doing some hand sanding.