there are lots of ways.
you can place rubble on top of the colony and it will spread to it in a day or two then just pull it apart and the frag is attached already.
you can also as suggested cut it with a razor, I prefer to use stainless steal scissors. if you cut it in the tank any tiny bit that floats around the tank will start a new colony which can be a PITA.
after I cut it I toss it into a frag tray I have in one of my fuge's. the frag tray is filled part way with rubble, the Xenia sink and attach to the rubble in a day then just move the frag to a spot in the grow out tank.
hth
kc