Grate the soap and add it to a pot with water. Heat and stir until the soap has dissolved.
Add the soap mixture to a large bucket along with the washing soda, baking soda, borax, and optional essential oils. Stir to combine.
Add more hot water, stirring until everything is fully dissolved.
Cover and let the mixture cool and thicken overnight.
Transfer to a container with a lid for storage.
Notes
It’s normal for this to thicken or gel as it cools. Stir before using if needed.This type of detergent is soap-based, so results can vary depending on your water.