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. (Put on your dust mask and let dust settle before opening processor, so as not to inhale it.)
Put on your gloves and dust mask, or cover your mouth and nose with a bandana.
In a large plastic tub or bucket, mix the Borax, washing soda, and baking soda together. (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. You will need to let the dust settle a few times before continuing to stir. If you can seal the container, you can shake it vigorously to mix with no dust.)
Thoroughly mix in the grated/powdered bar soap.
Stir in 10-20 drops of essential oil, if tolerated. Tea tree oil is great for diapers as it has antiseptic qualities.
Store in a covered, airtight container.
If your powder has trouble completely dissolving, try mixing it in a little hot water before adding to the laundry.
Use 1-2 Tablespoons per load. (Adjust for your machine.)