In a medium saucepot, combine baking soda and cornstarch. Stir in cool water and mix completely.
Cook over medium heat, stirring constantly. After 5 - 7 minutes the mixture will begin to brown slightly. It should begin bubbling. Continue stirring for a few minutes as it thickens.
Transfer to a glass bowl, and cover lightly with a damp cloth.
Let cool for at least on hour.
Once cool, knead the dough until smooth. Roll out to desire thickness. Use more cornstarch if needed to keep dough from sticking.
Stamp, or press the dough if desired. Cut with cutters. (Make sure to add a hole for hanging. A drinking straw works great for this!)
Let cool completely for 24 hours.