be prepared for the polyps on the edges of the frag to die.
just take a long heavy duty kitchen knife and chop straight down through the section you want removed. this will of course crush and mangle the polyps on the edges of the cut.
unfortunatly trying to baby the way through the coral by snipping tends to cause mis breaks and fractures horizontally killing many more polyps. this is one of the few times that a brutal fast
chop tends to be the most effective as it usually cleaves a nice vertical line damaging the least polyps. after fragging make sure that the coral and frag are in really good flow to was away the dying polyps and any infections, necrosis, etc, that will want to build up on the dead flesh, to prevent it from spreading to other polyps in the colony.
this is a moderatly risky frag. 1 to 10, ten being impossible 1 being mushroom its about a 6.5-7 (I have never seen a 10, goniopora rates a 9 on my scale)