Draw the shapes you would like to make on pieces of scrap cardboard. Stick to geometric shapes such as stars, pentagons, hexagons, diamonds and trapezoids. Each shape should be at least 12 inches across. With stars, the center of the star --- not counting the arms --- should be at least 10 inches across.
Cut out your cardboard shapes, using a utility knife. Tape flat pieces of cardboard along the edges of each shape at 90-degree angles to create boxes. Use several layers of tape on the corners of the boxes to make sure they don't leak or split apart.
Spread out newspapers and place a plastic tub on top of them. Pour stepping stone plaster into the tub. You can get this plaster in several different colors at hardware or craft supply stores.
Mix enough water into the plaster to make a mix that's the consistency of very thick oatmeal. Some plasters require specific ratios of water to plaster; read the ratios on the label carefully before adding water.
Pour about 2 inches of plaster into each of your cardboard shapes. Allow the stepping stones to dry for at least 48 hours. Peel away the cardboard boxes and place the stepping stones as desired.