Grate the entire bar Fels Naptha (or other laundry bar soap (Note 1) using the fine side of a cheese grater.
Pour grated soap, borax, and washing soda into a large mixing bowl.
Stir to mix well then transfer mixture to quart-size or larger container (Note 4). Apply the lid and label clearly.
To Use: Add 1 tablespoon powdered laundry detergent to the wash load. You may need to adjust depending on your conditions and washer size. You will not need much to produce excellent results.