In addition to the regular woodworking tools gather an assortment of files, rasps and rifflers. Small ones are handy for details; a few should have a safe edge. A small, sharp knife, or scraper [burr edge] serve well to shape, reduce thickness and clean up the surface of the horn. Rely on your own devices to hold the horn securely while being worked. A cloth bag filled with sand or small shot is useful, as is an 18 to 20" length of 3 x 3" [taper both ends; wrap one end in leather and sink a 3/16" iron rod or heavy framing nail -with the head cut off, into the other]. Mount this in a swivel or tail vise and use the stud end, inserted into the spout to work the throat, and the leather end inserted into the butt to rasp and scrape the body.
Hands-on and a little imagination will reveal the rest; or get the books.
~Longshot