Originally baking powder was made from baking soda (sodium bicarbonate) and creme of tartar (tatric acid), like this one is. For the chemists in the kitchen out there, the baking soda is alkaline and the creme of tartar is acidic. When the two are combined and liquid is added a chemical reaction happens, carbon dioxide is made and the product will rise (ie. acts as a leavening agent). Quite a simple process really. However as creme of tartar is quite costly, so the food manufacturing business took it upon themselves to find cheaper, alternative acids to creme of tartar. This is when aluminium based acid products were investigated and used as alternatives or in addition to traditional baking powder.
To make self-raising flour from plain flour using baking powder: For every cup of flour add 2 teaspoons of baking powder and mix well.