Grate the soap chunks with a fine cheese grater or throw the chunks into the food processor and blend into as fine of a powder as you can make. Wear your dust mask (or a bandana over your nose and mouth), and let dust settle before opening processor, so as not to inhale it.
Place grated bar soap in a pot. Cover with water and simmer over medium heat until all soap is melted, stirring occasionally.
Pour melted soap mixture into a clean 5-gallon bucket.
Put on your gloves and dust mask.
Add washing soda, baking soda, and borax to the soap mixture and mix well. (Washing soda, baking soda, and borax are mild skin irritants, so wear gloves. Wear a dusk mask or bandana to avoid breathing in the dry ingredients while you mix.)
Add enough hot water to almost fill the bucket, leaving 3-5 inches of headroom. Mix very well until all ingredients are dissolved. (I use a long ruler for this.)
If using essential oil, AFTER the mix has cooled down completely, mix in 10-20 drops, to taste.
Let sit overnight to gel. The gel will be loose and very gloppy—like egg-drop soup.
Use a funnel to pour the gel into clean, recycled detergent containers or leave in the bucket. Cover with an airtight lid if leaving in the bucket to protect children and pets.
Stir or shake well before using. Use 1/2 – 1 cup per load. (Adjust for your machine.)