that colony is awesome. personally I wouldnt frag it.
but that aside. you need something to make relief cuts so you split it how you want. a wet tile saw is usually best. brains are really finiky when it comes to fragging, you need a+ water conditions otherwise secondary infection will kill it after the fragging. once you make a relief cut or two to controll the split you can wedge the coral apart with a chisel and hmmer if you would like. you have to be careful not to squish any of the polyps as the brains skeleton will cut the polyps to pieces on the inside. once you have made the split you place the split pieces in a bucket of new well areated salt water with a powerhead pointed upwards for flow and oxygenation and a heater for temperature, leave it in the bucket for about a half hour to an hour for the coral to "slime off", then return it to your tank. discard the bucket water.