Make the mixture: Mix baking soda and cornstarch in a small saucepan. Add water and whisk until soda and starch are dissolved and the mixture is smooth. Add 5-10 drops of essential oil.
Cook over medium heat, stirring constantly, for about 5 minutes or until thickened. As you stir, scrape the bottom and sides of the bowl as well. The mixture will first get bubbly before it gets thicker.
Once the mixture is thick like toothpaste, remove it from the stove. Transfer dough to a cool plate or glass bowl and allow it to cool to room temperature (about 30 minutes). Cover it with a damp cloth to prevent it from drying out, if you're leaving it for longer.
Place dough on your work surface. Dust it with cornstarch and knead the dough into a smooth ball. If it’s sticky or too wet add more cornstarch, tablespoon by tablespoon and keep kneading until it’s less sticky. It should be pliable, but not sticky wet nor dry and crumbly. (It will break if it's too dry.)
Add more cornstarch to your work surface and roll out the dough to 1/4 inch (6mm) thick. Use a normal rolling pin for rolling. Only use a patterned rolling pin for a final decorative roll-out, if using.
Cut out desired shapes and transfer them to a baking sheet lined with parchment paper. Cut out holes (for hanging the ornaments) with a straw. Let ornaments air-dry overnight. Flip them over the next day and let them dry until they’re completely set, ideally another day. The longer you wait the better.