This food tip has saved me many times in the kitchen!!
How to make baking powder:
You need two ingredients - cream of tartar and baking soda.
Mix together 2 parts of cream of tartar, and 1 part baking soda.
An example would be 2 tsp cream of tartar and 1 tsp baking soda.
Combine and use immediately, this is not to be stored.
*If you want to make a little extra to store in the cupboard, add 1 tsp of corn starch to the mix. Keep in a tightly sealed container.
I would like to add that I make this substitution all the time, but I have never made any with corn starch. Therefore, I can not comment on how well it works. If you try it, I would be interested in knowing how well it works.