Mix: In a small bowl, combine 2 teaspoons of cream of tartar with 1 teaspoon of baking soda. This will give you a single-acting baking powder.
Add Cornstarch (Optional): If you plan to store your homemade baking powder for later use, add 1 teaspoon of cornstarch to the mixture. This helps absorb any moisture and prevents clumping.
Notes
Keep your homemade baking powder in an air-tight container. Use it just like you would store-bought baking powder in various recipes.