Combine the baking soda, water, and cornstarch in a medium saucepan. If you are going to tint the dough to the color of your choice, now is the time to also add the food coloring.
Heat over medium stirring constantly until the mixture starts to resemble mashed potatoes.
Remove & transfer the mixture to a glass bowl.
Cover with a damp towel & allow it to cool at room temperature for about 30 minutes.
Once cool, preheat the oven to 175 degrees.
Place the clay on a sheet of parchment paper and roll it out into a flat layer - about 1/4" thick. (if it's still sticky you can add a little more cornstarch)
Cut the dough into the desired shapes, stamp with any stampers you choose, and transfer the cut shapes onto a parchment-lined baking sheet.
Bake approx 30 minutes - gently flip them over & bake another 30 minutes.
Let stand at room temperature for at least 24 hours (or until completely dried out) before painting. (this may take longer depending on the humidity and temperature of your home)