"Recreating the 18th Century Powder Horn" by Scott and Cathy Sibley, is a terrific little booklet that I've found quite helpful. Track of the Wolf sells it online. If you are in eastern Pa., Dixon's sells it as well.
A word of advise from a voice of experience...drill the hole for the Spout first!! Nothing as disasterous as all that scrapping and sanding and shaping then to drill out through the side or bottom of the Horn. Lesson learned....
What's the preferred method of removing the bone from the horn?
It's a little hard at this time of the year, down right impossible, but let mother nature do it. Dig a hole in the ground, bury them, don't need to be anymore then 4 - 6 inches covering. Let them in the ground for a week or two. Dig back up, quick flip of your wrist, and inters slide right out.
The way that you wander is the way that you choose,
The day that you tarry is the day that you lose.
Sunshine or thunder, a man will always wonder.
Where the fair wind blows.