In the UK it is easy to buy flour that has already had the raising agent added to it. If this is not available you will need to add 1 teaspoon of baking powder to each 100g or 1/2 a cup of flour. To make 1 teaspoon of baking powder use half a teaspoon of cream of tartar and a quarter teaspoon of bicarbonate of soda.(The remaining quarter teaspoon would normally be a filler such as cornflour in bought baking powder.)