[header] How to make washing soda [title] Gather your supplies. [step] You will need a large metal baking dish (though stone or glass will also work), a one-pound (454 grams) box of baking soda, and a wooden spoon. [title] Preheat your oven to 400 f (204 c). 
[step] Cover the bottom of the baking dish with a thin layer of baking soda. Baking a thinner layer that's about one-quarter inch (0.6 cm) thick will ensure that the process occurs uniformly.