SUBSTITUTE FOR BAKING SODA: USING BAKING POWDER INSTEAD OF BAKING SODA
You need to use 2-3 times more baking powder than baking soda. The extra ingredients in the baking powder will have an effect on the taste of whatever you are making, but this isn't necessarily bad.
Ideally, triple the amount of baking powder to equal the amount of baking soda. So, if the recipe called for 1 tsp baking soda, you would use 3 tsp baking powder.
One option is to make a compromise and use twice the amount of baking powder as baking soda (add 2 tsp of baking powder if the recipe calls for 1 tsp baking soda), plus I omit the salt (which adds flavor but also affects rising in some recipes).
SUBSTITUTE FOR BAKING POWDER: HOW TO MAKE IT YOURSELF
You need baking soda and cream of tartar to make baking powder.
Mix 2 parts cream of tartar with 1 part baking soda. For example, mix 2 tsp cream of tartar with 1 tsp baking soda.
Use the amount of baking powder called for by the recipe. No matter how much homemade baking powder you made, if the recipe calls for 1-1/2 tsp, add exactly 1-1/2 tsp of your mixture.
Cream of tartar is used to increase the acidity of a mixture. So, you can't always use baking soda in recipes that call for baking powder.
You can switch baking powder for baking soda, however, just expect the flavor to change a little.