Sir,
It is not as difficult as you would think. Polish the horn, draw your map on the surface with a soft lead pencil, coat with acrylic protector (art store), scratch the design onto the horn using a sharp object (a light touch is better than a heavy hand), coat with India ink, remove excess ink with damp paper towel, steel wool to remove acrylic and finally coat the horn with wax. Done!
You'll be surprised at how well it will come out. Just don't be in a hurry. Start with the main out-line first, then add the detail. The toughest part is getting the initial design the way you want it on the curved surface. Make sure to use your eraser, and get it how you like it. Mistakes can usually be scraped out with a sharp blade. Make sure you have good lighting, as it makes it easier to work.
Hey, if the old guys could do it on a whale tooth polished with oil and ashes on the "bounding main", you can sure do it!!!