You do not want to boil them, heat them dry in the oven instead. Here's the problem. If you boil them you completely saturate the horn with moisture. Then you need to wait a year for them to cure again. That's the reason people have trouble with horn.
First, cut the tip off for a suitable hidden tang handle. Then cut off the outside strip (around the outside curve of the horn) thick enough for scales. Then split the two sides of the horn. You should get three strips from each horn like a triangle, unless they are too narrow, then skip the first cut.
Heat your household oven to 350 degrees. I have my own oven in the shop for this. Place the horn on the rack and wait. Do not answer the phone or get distracted. After about ten minutes check the horn with oven mits on. Check them often after that. When they are as flexible as a car tire (with no air in it) they are ready to press. Take them out and clamp them between two pieces of ply wood. Let them cool over night.
Here's a neat trick for something unusual. You can leave them in the oven just a little longer to change the color. Just after they are flexible enough to bend they will start to turn orange or butterscotch colored. Very shortly after that they will start to burn so they have to be taken out quickly after that.
I suggest doing them in small batches so they don't get ahead of you. Have all your clamps and boards ready ahead of time.
Everything Ed said is true, they like to move. Once they have cooled, they shouldn't be subjected to heat or moisture any more. Work them slow, heat from friction with warp them. Use sharp tools and abrasives. Do not dip them in water if they start to get warm. Put them down and do something else.
Dovetail your bolsters, that helps keep them from curling. Use corby or Loveless rivets when you can. Do hidden tangs when you can, like on the horn tip. All these things will lessen the chance of handle failure.
All that having been said, I have had several hundred knives out, starting 18 years ago with sheep or musk ox horn on them and have had only two come back. Those were musk ox horn, notorious for shrinking.
When the knife is finished it can't be left on the dash of a truck on a hot day or displayed on a fire place mantle. Keep cool and dry.
Good luck, have fun and go with confidence my friend.