Put the soda ash and baking soda in a food processor to combine.
Mix the castile soap and water together in a separate bowl and pour slowly into the powders with the processor is still running. You should get a white powder.
Add the essential oil and continue to process until mixed.
Recipe by Eco at Heart at http://blog.ecoatheart.com/deadly-detergents-top-10-toxic-laundry-ingredients-diy-laundry-powder-recipe/