Start out with Sibley's book as Soggy suggested. If you really get into this horn building, study the originals from books like Powder Horns and Their Architecture, by Madison Grant, and The Engraved Powder Horn by Jim Dresslar. One thing you will find is that when you engrave your horns, you'll make mistakes with your scrimshaw. The horn has hard areas and soft areas, and you will slip. Don't get discouraged. One thing I found by studying original engraving on horns, is that I could identify where the engraver had made a mistake, as I had made the same mistakes!
Scott