This project can be made at the beach right there in the sand or in a large bowl filled with moist sand. If sand is dry, mist it with a spray bottle, or sprinkle it with handfuls of water. Get the sand evenly moist by spreading and mixing with your hands. Form a 6-inch diameter circle, about 2-inches deep, this will be your mold. Smooth the floor of the mold with your hand and pat it down gently.
Place the twigs into the circle, pressing down firmly, but gently, forming a starfish. You do not want any plaster to seep underneath them.
Arrange the sea shells face down in the sand, pressing in firmly, but gently.
Mix 3 cups of plaster with 1 1/2 cups of water in 4-cup measuring cup. Stir mixture well until creamy, be sure to remove all lumps. Slowly pour the plaster mixture into the sand mold. You do not want to pour it too quickly or you may disturb the placement of your objects (see tips at the end of these instructions for disposal of your remaining plaster).
Allow plaster to dry for at least an hour to let it harden.
Carefully dig around the edges of your mold with the spoon and gently loosen and lift the plaster cast.
Let the sand casting dry for another hour. Use stiff bristled paintbrushes to remove the excess sand.