Use a food processor or grate the bar of soap.
In a large pot or container, combine 8 cups of water, the washing soda, and the borax. Mix well.
Add the grated soap or liquid castile soap. Simmer on low heat for 20 minutes, stirring occasionally. Make sure all the soap is fully dissolved.
Remove from heat and stir in your desired optional essential oils.
Pour 4 gallons of water into a clean 5-gallon bucket.
Pour the soap mixture into the bucket with the water.
Stir vigorously for a minute or two.
Cover the bucket and let it sit for 24 hours before using it.
Use a cup per load added directly into the drum of the washer, not the soap dispenser tray. Use 1/2 the amount for a high-efficiency washing machine.