I have seen someone frag them on youtube, he just cut them into quarters, making sure he cut right through the middle of the mushroom each time, I have no idea how successful it is though.
The best way to frag a mushroom it to take the rock out of the water and remove the mushroom and a little sliver of the rock using a chisel. ****Please wear safety glasses as they can squirt and id also advise wearing rubber gloves when fragging.****
Once you have your mushroom with sliver of rock attached you can take a scalpel or stanley blade and cut it into quarters through the mouth and thats your fragging done.
The idea of having the little sliver of rock still on the mushroom is so you can super glue it onto some frag plugs or onto live rock rubble. Make sure the superglue you use is cyanoacrylate based.
If your mushroom has come loose and you dont have the sliver of rock to superglue you can get something like a well washed butter tub and put live rock rubble in the base and set your frags on top, then cover with fine mesh and set in a low flow area of your tank for a week or 2 til the frags become attached to the rubble.
oh thats easier to make small mate lift it out and cut the outside ring off it with a stanley blade.
to make frags to swap with other reefers you will need to cut the ring into sections and pierce it with a cocktail stick and tie the cocktail stick to a frag plug or live rock to let the frag attach