From Wellness Mama.
Heat oven to 400° F. Pour baking soda into a baking dish (I use a glass Pyrex casserole dish) about 1⁄2 inch deep. Bake in the oven for ~1 hour, or until the consistency is right; stir a couple of times. Washing soda tends to feel “grainy” rather than “silky”. When cool, store in your container of choice.
From Hadria 