I just realized I posted this recipe under my pancake recipe, but I thought I should also post it separately:
1/3 cup baking soda
2/3 cup cream of tartar
2/3 cup arrowroot or tapioca starch
Mix three ingredients, store in air-tight container. As long as you do 2 parts arrowroot or tapioca and 2 parts cream of tartar to 1 part baking soda, you'll be fine. The 1/3, 2/3 Cup is just the amount it takes to fill our container.