Microwave the bar of Castile soap for 30 seconds. Turn the bar over and microwave for an additional 30 seconds. Repeat for an additional 30 seconds on each side.

Remove the softened bar from the microwave and coarsely cut it up into pieces while it is still hot.

Cool the cut soap pieces for 5-10 minutes.

Finely grate the still-warm, coarsely chopped soap pieces into soap flakes in a food processor.

Mix the borax, washing soda, and soap flakes in a half-gallon container. Close the lid tightly and turn several times to ensure it is blended thoroughly.

Use 2 tablespoons of this DIY laundry powder per medium load.