A basic recipe for homemade laundry detergent powder is to mix 1 part washing soda, 1 part borax, and 2 parts grated soap flakes (for instance, for every cup of washing soda and borax each you will need 2 cups of grated soap flakes).
Use a large cheese grater to grate a bar of soap for the flakes. If you use Fels Naptha soap, which is quite harsh and not recommended for use on the skin, use half the amount of soap per batch (that is, equal parts of each ingredient).
Mix ingredients well and store in a plastic container with a tight fitting lid. For large loads in a top loading washer, use 2 to 3 Tablespoons of the detergent. You can add easily add a scent to your laundry by adding just a few drops of any essential oil to the water in your washing machine each load of wash you do.