Go to the Track of the Wolf site- see links up at the top of the index to this forum, and look for the Book Recreating the 18th Century Powder Horn, by Scott and Kathy Selby. But it. It has everything you need to know to work horns.
Paul is correct. The Sibley book covers it all. My most used tools for shaping a horn is are ******* files, and round files. A chainsaw file is good. I also use sandpaper, 220 and 320 grit. Some horners will use scrapers.
Scott