Put the arrowroot (or arrowroot and cocoa powder) in a small jar: 4 TBSP arrowroot for light hair, or just 2 TBSP for dark hair, with the cocoa.
Add the essential oil and stir to blend.
Notes
Apply to roots or oily areas with a makeup brush (or sprinkle and comb through), then style as usual. For dark hair without cocoa, apply plain arrowroot the night before or at least 2 hours ahead. Spray version: add 1 cup warm distilled water and 1/4 cup vodka, rubbing alcohol or witch hazel; shake before each use. Safety: avoid breathing the powder and keep it out of eyes; cocoa powder is toxic to dogs, so store out of pets' reach; keep essential oils kid- and pet-safe.