Transfer the homemade baking powder to a Mason jar and store with a tight-fitting lid.
Notes
Variations on homemade baking powder recipe:
the above homemade baking powder recipe comes from Paula Figoni's How Baking Works (available for purchase from Amazon) and I have done the math to make sure that the amount of cream of tartar and baking soda will balance out correctly.
Shirley Corriher in the book Bakewise recommends a different ratio, specifically: 14 grams baking soda + 18 grams cream of tartar + 15 grams cornstarch. Her book is also available on Amazon. Chemistry-wise, I don't think this ratio adds up. I wouldn't follow Corriher's recipe for baking powder.
Alton Brown suggests a ratio for homemade baking powder in his book on baking called "I'm just here for more food" but he doesn't mention if the ratio is by weight or by volume (so use with caution). After more research, I found the same recipe in the textbook "On Baking" which mentions the recipe is by volume: 2 parts cream of tartar, 1 part baking soda, 1 part cornstarch. Alton Brown's baking book can be purchased from Amazon and the textbook On Baking is also from Amazon. To me, this ratio is also flawed. I would still stick with the recipe from Figoni's How Baking Works.