Gradually add enough flour to make a stiff dough. Place on a lightly greased cookie sheet. Bake at 325 until browned and hard. Put plaster, paint, water and soap in a bowl and mix well. Pour into molds, and let dry several hours or overnight, depending on the size of the mold.
Molds can be anything but the perfect size is an inexpensive popcicle tray.
Small dixie cups work well as well. Try layering different colors in the molds to make neat looking chalk. Pour boiling water slowly over cornstarch mixture, stirring. Cook over medium heat, stirring constantly, until mixture boils looks clear. Remove from heat.