Well, never done it myself yet, but I saw how it was done on e. g. Acans and Catalaphylia's. They used a diamond saw to cut the block of Acans apart into smaller frags. Most important is to keep the polyps complete so that each has a mouth. I guess the same goes for the hammer coral. Since they grow in branches it should be easy to cut or carefully break a branch off and place it on a frag stone. But as mentioned, be very careful since their coraline skeleton is very fragile, very thin and breaks very easily. Also, be careful not to damage any body tissue. Some LPS are able to regenerate tissue, but most can't or very hard. Guess best would be to tease it until it retracts almost completly and then frag it. This way you should be able to see where you can break/cut off a branch best, too.