Baking Powder is
1 part baking soda
2 parts cream of tartar
So if you need 1 T baking powder (1 T is equal to 3 tsp) use 2 tsp cream of tartar and 1 tsp baking soda.
Don't make a huge mix of homemade baking powder ahead of time, make it as you need it for the homemade kind doesn't having shelf stability.