super easy.
take a clean blade and slice it in place. try to get a piece of mouth in each 1/2 or quarter(depending on how big it is) and put it back and let it heal on its own.
this is the best way IMO as you dont have to deal with reattaching it ta anything as each part will already be attached.
if it is a free floating polyp you can do the same thing then just put the parts in a little tupperware container riddled with holes and filled with rubble until they attach to the rubble.