Use 1/4 teaspoon baking soda for every 1 teaspoon baking powder
Add an acid to activate the baking soda
Use 1/2 teaspoon cream of tartar for every 1/4 teaspoon baking soda
Or use 1/2 cup buttermilk, yogurt, or sour milk for every 1/4 teaspoon baking soda
Reduce other liquids in the recipe if using a liquid acid
Bake immediately after mixing
Do not substitute baking soda alone for baking powder without an acid source