reefmutt-
I just did this exact same thing for the first time about a month ago. So I can tell you what I did.
I took my mushroom rock out of the water, lifted up the mushroom so that I could get at the stalk, and then sliced it with a razor blade. I then cut the mushroom into quarters, trying to get part of the mouth in each piece (didn't quite succeed with that).
I did this with three mushrooms.
Then I took the mushroom pieces and put them in a plastic cup that I had put some gravel in, and set it on the substrate inside my tank, in such a way that it wouldn't tip over. After about a week and a half, the pieces had attached to some pebbles.
I took the pieces that had attached to pebbles, and superglued them onto the rock that I wanted them on.
It's been, oh, about 4 or 5 weeks now, and they all survived, and are showing growth. Also, new mushrooms are growing where I cut the old ones off.
BTW, don't be alarmed when some really weird looking stuff comes out of the mushroom when you slice it. CRAZY SLIMY.
I read a LOT about this before I tried it, and it seems like you just can't possibly go wrong propping shrooms. Just don't want them to float away and attach where you want them.
HTH.