This is how I made a shell buttcap for a reproduction Bowie.You can find these that have been casted or carve one yourself and have it casted for you.
Hope this helps ,Bruce
Start with either a solid piece of metal or like I did here Hard solder 3 pieces together
Mark where you want the small shoulder part of the shell.Then with a metal cutting saw of some sort cut the corners out
After cutting the corners out.Get a 3 corner or flat file and carve in the sides to the depths you want matching the corners and ovaling  the shape some.Make this  the size you are going to make the front ferule in front of the handle
As you see here I left a cutout in the center piece for the tang to go in.You can drill and tape this section or inset a nut and solder it in or you can just put notches and a hole in the tang and epoxt it on.....Now go to the grinder and give it the outside shape you want and mark where you will need to file in the grooves
All you need here is a couple different sized 3 corner files one to cut the big groove and the small one to clean up and round the edges off.Cut your grooves in one side then turn the piece up and cut across the edges and then cut the other side
After cutting in the shell shape like this you need to cut a groove all the way around the oval part of the base(in the center).......Now with a file and sandpaper clean up the grooves and round the sharp shoulders to your liking.......Now take it to the buffer and carefully buff it and let it wash the edges to a soft round
The finished shell on the Knife
Just give this project a try and it might shock you at how easy it really is.and it sure does dress up a knife handle nicely............Bruce