A local fellow reefer just showed me how to frag my large toadstool.
You don't have to take it out of your tank. Just use very sharp razor blade and slice 1-2" from the edge whichever part you want to frag, which in my case gave me a big size frag.
He, then, took the frag out of the tank, then sliced that frag into two to make a reasonable size frags.
Glue two pieces of rubbles to make sort of a letter V or L; Pierce each frag with a toothpick (kind of making a kabob of toadstool), then glue each end of the toothpick that has the frag to the V shape rubbles. This technique secures the frag so it won't get blown away with the water current.
Put the frags in a good flow area to avoid any infections. My frags had already attached into the rubbles within 2 weeks.
Good luck!