In a medium bowl, add 1 cup of Borax and 1 cup of washing soda.
Cut the Fels-Naptha into chunks and run it through a food processor until finely grated. You can also grate the soap, but if you wash some clothes in cold water, it may not dissolve well. Plus, doing it in the food processor is much quicker!
Add the Fels-Naptha powder to your bowl with other ingredients.
/2 cup of Downy UN-Stopables or Purex Crystals, (if using). You can run those through the processor too. Then add to the other ingredients.
With a spoon, mix everything together very well. Then store in your chosen container.