Remove it from the tank, get a scalpel or very sharp knife and try and cut as clean a cut as you can. I assume your work on cutting the "fingers" first? Just make sure you do this outside of the tank and that you have a place to put the mother coral and the frags for a while, until they stop sliming, which they will.
To attach the frags, you can either use a bunch of rubble in a tupperware container (lay down a layer of rock rubble, then lay the frags on top in a low flow area. Give them a few days/weeks and they'll attach themselves) or, you can use the toothpick method (take a toothpick, stab the frag at the base, then attach the toothpick to rubble or a rock in the tank.