I took these three photos just in the last week:
Then, choose a modeling material. I made this craft with both white Sculpey (though any bake-able polymer clay will do) and plain Paperclay. The main benefit to the Sculpey is that you can paint your mushroom soon after you bake it. Paperclay is a bit easier and softer to handle and sculpt, but you'll have to wait at least 24 hours (and sometimes longer) for the clay to dry before painting, and waiting is difficult for antsy kids or impatient grown-ups.
Using either modeling clay, start by pinching off a blob and pressing it into the underside of a clean, dry acorn cap.
Here are two mushrooms I sculpted out of Sculpey. To bake them, lay them on their sides on a foil-covered baking pan. Don't worry: the temperature at which the clay bakes isn't high enough to scorch the toothpicks or acorn caps.
Here's another angle that better shows how I painted the undersides of mine. Again, I went fancy, because sometimes I do that.