Horn may be shaped rather easily after boiling, or using a heat gun. It becomes like soft plastic when hot. Depending on the thickness, and even age of the horn, will determine how long to boil. I would suggest orienting the buttplate on the wood and align the screw holes. Boil the horn and screw it in place, it will conform to the shape of the butt. It will retain that shape after cooling and/or drying. The edges may be filed using a bastard cut file, or a cabinet-makers rasp. To thin horn you must scrape it. Use a sharp knife edge at a little less than 90 degrees. It will come off in thin slivers. It takes time and you must be patient.
