Combine all the ingredients in an old shampoo bottle or jar. Pump soap dispensers and even foaming dispensers work well for this. If you use a foaming dispenser, add ¼ cup of distilled water.
Shake well to mix.
Shake before each use and use about a teaspoon every time you shampoo.
Store in the fridge for up to a week.
Notes
If you use a foaming dispenser, it also makes a great shaving cream.
Some people find that a 50:50 mixture of apple cider vinegar and water used as a rinse helps get hair cleaner and increases shine.