Are you looking to completely frag it up? Or, do you just want to give it a trim?
If you want to completely frag it up, you could cut the cap off where it attaches to the stalk. The stalk will grow a new cap, and the cap you removed can be cut into numerous frags.
If you're just looking to give it a trim, the "donut" or "ring" method works pretty well without distorting the shape of the cap. Essentially, you cut a ring out around the entire outer edge of the cap, then cut that ring into numerous frags.
However you frag it, it's important to make sure the frags and parent receive enough flow. It'll help with any sliming and keep anything from settling upon them while they are trying to heal.
I frag our toadstool leather frequently. There's really little to be worried about. I've used a pair of stainless steel scissors to cut frags off (which I find very easy), however, a clean razor blade works very well, too (thinner cut without smashing any of the tissue).
I prefer using toule / wedding veil material to hold the frags in place until they heal. I wrap the material completely around the frag and plug or small piece of rubble / shell I want the frag to attach to, then rubberband the material in place. It takes a week or two, but the frag will attach on its own and start growing a stalk. Super glue is more difficult to use because of all the sliming, and I've had many toadstool frags that seemed secure come free after a few days. I'm not a fan of the rubberband and toothpick method for securing frags simply because I don't like the idea of putting a hole into the frags. Yes, it's effective, but it's not my choice for fragging leathers.
