Combine 1 cup cornstarch, 2 cups baking soda and 1 1/4 cups water in a pot.
Stir the mixture over low heat. The stirring keeps the ingredients from sticking to the pot and burning. Continue to stir and cook the dough until it looks like mashed potatoes. It shouldn't be soupy.
Remove the pot from the heat and wait a few minutes for it to cool slightly. Use a spoon to gather the mixture into a blob.
Turn the blob out onto a floured surface, and as soon as it is cool enough to handle, knead the dough until it's nice and smooth.
After you have created your artwork, allow it to dry for several days at room temperature. You can then paint, lacquer or draw on the finished product.