Mix all ingredients in a glass bowl with a wooden spoon. Avoid bringing the clay in contact with metal. The mixture should be smooth and easy to spread. If too thick, add more water; if too thin, add more clay.
Wet hair, then distribute mixture evenly through hair and scalp with your fingers. Massage scalp and let the mixture sit for about 5 minutes.
Rinse thoroughly with warm water. Optional: do an extra rinse with 2 cups of tea.