Fabian,

I have not tried to frag my shrooms, but this is what I was told to do

Which leads me to a method that has worked 100% of the time for me (to date). I just cut the head of the mushroom off leaving some stalk behind. In a couple of days, the stalk closes over itself to heal the wound. At this point, I make one cut completely through the middle of the stalk. Both sides will heal over leaving two stalks which will, in turn, each grow another head. I have not tried dividing the stalks into more sections (haven't been that brave), but that may work as well. Hope this information works for you.
or
cut the mushroom into four pieces like a pizza pie. After cutting them and let them heal first, until they are fully healed, in my opinion, when the disk is fully form, this will lead to stronger cuttings.
When the cuttings are fully formed remove all of then with exception of one, this will provide more cutting in a few weeks. The cuttings should be removed with either a sharp razor blade or scissors.

Make the cut bellow the disk as close as possible to the base, these cuttings are much stronger than cuttings done of just the disk of the mushroom. After the cuttings have been removed, dip then in a small cup of the aquarium water with a few drops of iodine for about three minutes.

I found out that after about two days that most of the cuttings detached by themselves. Once this is done, put the cuttings into a gravel bowl
sorry, the info is not mine, but I hope this helps.

Adam in Austin