Grate the Fels-Naptha soap using a cheese grater or food processor. You want it finely grated to ensure it dissolves easily in the wash.
In a large bowl or container, combine the grated Fels-Naptha soap with the Borax, washing soda, and Oxy booster. Mix well to ensure all ingredients are evenly distributed.
If desired, add the Purex crystals to the mixture for a pleasant fragrance. Mix thoroughly to combine.
Transfer the mixture to an airtight container for storage. A large mason jar or plastic container works well for this purpose.
To use, add approximately 2 tablespoons to each load of laundry. You may need to adjust the amount depending on the size and soil level of your laundry.
Store the homemade laundry detergent in a cool, dry place away from moisture and direct sunlight to prevent clumping.