On a store bought polished horn: Coarse rasp, like a 49 Nicholson, half round Nicholson Bastard, scraper with NO burr.
On a raw horn start with a coarser rasp like a 50, then as above. Stay away from sand paper, buffer, polish or burnishing. Of course a lot depends on what you want the horn to look like, aged, new, used, abuse, neglected, etc…
The only want to get what you want is to experiment. I don’t think anything you do to a horn cannot be corrected, unless of course you cut it in half and even then the parts are still useable.
Tim C.