Combine baking soda, corn starch and water in saucepan over medium heat.
Stir constantly until mixture resembles mashed potatoes (3-5 minutes). Note: a Danish dough whisk works GREAT for really mixing up this dough.
Transfer dough to your work surface and cover with a damp cloth. Let cool until room temp or warm (not hot).
Knead cooled dough until smooth.
Roll dough out to ¼" thickness. Cut out shapes using cookie cutters.
Cut out a hole for the ornaments by poking a straw into the dough.
Transfer ornaments to a baking sheet lined with parchment paper. Allow ornaments to dry 24-48 hours, turning over every 8 hours or so for even drying.
Paint ornaments with acrylic craft paint.
Add string and hang on the tree!
Video
Notes
If the dough is too sticky while kneading, add a small amount of corn starch to your hands. You want the dough to be nice and soft to prevent cracking, so don't add too much. Just enough to be able to handle it without the dough sticking to your hands. I usually don't need to add any extra corn starch.