Use 1/4 teaspoon baking soda for every 1 teaspoon baking powder
Add an acid to the recipe to activate the baking soda
Use 1/2 cup buttermilk, yogurt, or sour milk for each 1 teaspoon baking powder replaced
Reduce other liquids in the recipe by 1/2 cup for each 1/2 cup acidic liquid added
Use 1/2 teaspoon cream of tartar plus 1/4 teaspoon baking soda for each 1 teaspoon baking powder replaced
Use 1 teaspoon lemon juice or vinegar plus 1/4 teaspoon baking soda for each 1 teaspoon baking powder replaced
Mix and bake immediately after combining the ingredients