(you will need four ounces of soap for this recipe). You can grate your soap by hand or in a food processor. A food processor is the fastest approach; 2. Bring 3L of water to a boil on the stove. 3. Then, turn off the heat, and add your soap flakes. Stir to melt the soap. Continue stirring until the mixture is fully combined. At this point, the mixture will be very liquidy; 4. Add the washing soda and alcohol; 5. Allow the mixture to cool and put the essential oil.