In contemporary terms, Castile refers to any vegetable oil-based soap, although you should to check labels carefully to ensure there are no artificial or petroleum-based additives. The original Castile soap was an olive-oil based soap made in the Castile region of Spain. Fans of Castile soap use it for a variety of purposes beyond washing hands. Because it's a natural, biodegradable product, pure Castile soap is used for cleaning household surfaces, clothes washing, degreasing, washing dishes -- and even as a substitute for shampoo.
Wet your hair and scalp thoroughly.
Squirt a few drops of castile soap onto your hands. A little bit of pure castile soap goes a long way, so start with this limited quantity; add more as needed, depending on how long and thick your hair is.
Rub your hands together briskly to work up a rich lather.
Work the lathered castile soap into your hair thoroughly, from scalp to ends.
Rinse your hair until the water runs clean.