Not to sound like a broken record, but Sibley's book is the one to get. A bit of advice I would offer is don't be afraid to carve on the horn. You'd be surprised on how much material there is to work with. Get a small flashlight, a maglite or gooseneck light. As you're working the horn putting the light down in the horn will enable you to see how much thickness is left. When shaping the horn, I like to rough it with a mill file. For final shaping and smoothing I use sandpaper. Take about a 1" strip and back it with duct tape. With the horn secured use the sandpaper like you're polishing a shoe to get an even surface. To get all the scratches out, cover the horn with wood stain (ink will also work) and use finer grit sandpaper, say 300 grit, working with the grain of the horn. Follow this up with 0000 steel wool. Others may have other techniques, I know some like to scrape the horns like the originals were. Let us know how you do, and have fun!
Scott