You can use plaster of paris, but I wanted to make my plaster starfish virtually unbreakable, so I went for the reinforced (with fiberglass) stuff, called Hydrocal Casting Plaster. I paid $9 for a bucket at a local art supply store. You can buy it online at places such as Blick Art Materials.
Before you prepare the mixture, coat the mold with the mold release spray used for making the mold. Important! To cast one plaster starfish you'll need about three teaspoons of powder. Slowly add water and stir until plaster is thick and even but thin enough to pour. I used a sauce dish!
Wet plaster has a very short working time, only a few minutes.
Shake the mold gently and tap it to bring up air bubbles. Fill in more plaster if needed, shake and tap again. Then let dry for a couple of hours. Also, make sure to clean dish and spoon immediately after casting. If you are planning to Hang Starfish you could insert a wire or twine. when dry, take out the starfish by carefully bending the mold.
I did smooth out the edges a bit on the backside of the starfish with fine sandpaper.
Now I look forward to grow my cast plaster starfish collection, decorate and craft with them. Here are a few ideas I like: Hang Starfish, Creative Displays and maybe I'll make a Starfish Wreath too. And of course you could paint them in any color you wish!