I have only used white tail antler for tip overlays but I'm sure the same principles apply. I cut a section slightly larger than the overlays intended final size with my bandsaw. a hand saw is ok but take great care to keep it straight and even. cut the piece in half thickness wise and then you have both tip overlays. sand the back of them with 120 grit sand paper and sand the bow tips where the overlays will rest LIGHTLY. clamp the antler/horn with wide faced clamps for good pressure displacement over the entire overlay. shape the tips after the glue has dried. clamp the bow limb near the tips for a a secure hold.
Thats my method, roughly, so take it with a grain of salt. good luck, I bet that bison will look great!