i just use 80 grit self adhesive sand paper on a block of wood to get the flat spot...
But wait.... There is an important part about getting those antler tip overlays to stay put. The antler material is very porous, and just soaks up glue. It's very important to seal that flat surface well before you try and glue them to glass or wood. You can use epoxy, or thick super glue to do it. then sand again before installation.
I had a lot of antler tip overlays pop off until i figured that one out... Food for thought. Kirk