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.
Dr. Bronner's Magic All-One recommends using a hair rinse and leave-in conditioner after shampooing with Castile soap, but users report that these steps may not be necessary.
Since you're likely to be using your Castile soap for many other uses around the house, store a little bit in a travel-size squeeze bottle and keep this near the shower for shampoo use. Refill the bottle as needed. This keeps you from having to run to the bathroom to retrieve the soap every time you want to use it for something else.