I take the section of horn that I am going to cut the ring from and draw the design on it. I then slide the section over a dowel locked in my vice. With files and a saw I cut the design in, spinning the piece as I go. When I get pretty close to done, I cut the ring out with a hacksaw. If needs be I scrape the inside down. Then I boil it in water, slide her on the horn and pin it on there.
Coryjoe