HOW TO MAKE CONCRETE MUSHROOMS. Dig a hole in the ground and line it with plastic. Fill the cavity with cement. Reinforce with a piece of window screen.The stem is formed with a cut off 2-liter plastic bottle. Isnt that clever? When cement is dry, tip it over and stand it up and there you are. Another pinner pushed in a PVC pipe into the stem to create a post that you sink into the ground to stabilize the mushroom.