Mix the baking soda and cream of tartar together until well combined. This will give you one Tablespoon of baking powder.
To store baking powder: Add a teaspoon of corn starch to the mixture, and stir. This will absorb any moisture from the air, and prevent the baking powder from clumping or reacting before you need it. Store in an air-tight container between uses.
Want to make a bigger batch? Just stick to the ratio of two parts cream of tartar to one part baking soda, and it'll come out great.
Since baking powder loses its potency over time, it's best to make no more than you can use in a month.