You are lucky that you need to remove material to make the handle more comfortable, not add same. With my large hands the HI handles are typically too small.
I have somewhat limited direct experience in working with horn, but from my research I can recommend the following: Keep it wet and cool when working if possible to keep down the smell and possiblity of delamination (a lot depends on the tools you have available, I have a wet sander used for glass processing that works quite well). Other than that it is a pretty soft material and easy to work. If you are trying for a high polish on your final handle be careful to switch directions on sanding so each successively finer grit shows the scratches from the previous one.
Good luck.