Here is an excerpt off of the Garf website on propagating (fragging) mushrooms. Hope it helps!
"When propogating mushrooms, I follow the same exact procedure as on your web site. 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".