Put hypertufa mushrooms into search space on this page and discover 69 threads on the subject. There isn't any single way of doing it that is right - but rather a number of ways. If you can stand to read those threads, you will get a sense for some ways that might work for you.

Cut the bottom out of a plastic cup, then slit one side of the cup- top to bottom. Curl up the cup to form a slightly conical stem mold--duct tape in place.
Fill with mix to make stem with a piece of dowell down the center to make a hole. Let stem cure until hard.
Get several diferent sizes of plastic bowls that you think look like upside down mushrooms. Line the bowls with thin plastic and fill with mix. While this cap is stil wet, wet the stem and insert it into the cap an inch or so with the loose dowel pushed up through the stem and into the cap, and purposely try to bond stem and cap together. If they don't bond together very well, the indentation in the cap can sit on the stem and the dowel will run from in the ground throught the stem into the cap. If the two bond well, the hardened the stem and cap should be one. Wire brush the cap to give the texture and look of a mushroom.