Mix the baking soda and cream of tartar together until well combined. Use immediately.
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 reacting before you need it. Store in an air-tight container.
Note: Most commerically-produced baking powder contains aluminum--sodium aluminum sulfate to be exact. Make your own baking powder, and keep your baked goods aluminum-free. Some people are concerned that aluminum causes/conributes to bone loss and Alzheimers. I will not try to argue either way, my grandmother had cheap aluminum pots and cast iron skillets and granite ware baking pans, she died at 88 from cancer and no dementia or never a broken bone. However, my aunt died at 70 something and used a fork as a comb and did not know her own name and several broken bones, she ate my grandmothers cooking but had stainless steel once she had her own home. Sp come see come saw as my family says.