Combine baking soda, cornstarch, and water in a medium saucepan. Stir over medium heat until mixture begins to thicken.
When it looks smooth and thick enough to hold it's shape, remove from the heat.
Transfer the clay to a bowl and cover with a damp towel or plastic wrap. Allow to cool.
Once cool, sprinkle cornstarch on a work surface and knead until smooth. If you wish to add food coloring to the clay, divide the clay into portions and knead in your food coloring.
Roll clay to 1/4" thickness, using additional cornstarch as necessary to prevent sticking. Cut out shapes with cookie cutters. Alternatively, you can just cut circles with a biscuit cutter. Use a straw to make a hole in the top of the ornament for hanging.
To bake: Place ornaments on a cookie sheet and bake for 30 minutes in a 175 degree oven. Turn the ornaments over and bake for an additional 30 minutes.
To air dry: Place ornaments on a cookie sheet and allow to dry overnight. Turn the ornaments over and continue to dry until hard.
Once these homemade clay ornaments have dried, you can paint them with acrylic paint if you choose. If you wish to add essential oils to the ornaments, be sure to leave one side plain. After they are completely dry, add 1-3 drops of your favorite essential oil blend.