I agree with all the advice, and would add that a Dremel tool cutting wheel is pretty useful in doing fancy carving around the decorative ring edges....also, I use jeweler's rouge on a wheel after I've done the fine scraping, and then use a paste wax and buffing wheel...this coat of wax also helps when scrimshawing.
To get a horn that is transparent, so you can check the powder level, I use a sanding drum in my drill press and sand the inside of the horn butt for a depth of about an inch and a half...this allows the horn to be held up to the light, butt up, and you can see if the powder level is below the level of an inch below the butt...Hank