What's the best way to use this oil as a pre-wash hair treatment?

Because castor oil is dense and viscous, applying it straight to dry hair works best as a targeted pre-wash treatment rather than a leave-in. Section dry, unwashed hair and dispense a small amount — start with a few drops for shoulder-length hair, more for longer or thicker hair — into your palm. Rub hands together to warm and slightly thin the oil, then smooth it over mid-lengths and ends where dryness and split ends concentrate. Avoid the scalp unless you have specifically dry scalp patches; the oil is hard to rinse from roots without multiple shampoo rounds. For easier distribution, mix castor oil 1:1 with a lighter carrier oil such as jojoba, almond, or grapeseed oil. Leave the treatment on for 30–60 minutes; some users cover hair with a warm towel or shower cap to help penetration. Shampoo thoroughly — you may need two rounds — then condition as usual. Hair should feel softer and look glossier once dry. Frequency: once a week for very dry or damaged hair, every two weeks for maintenance. If hair feels weighed down or greasy after drying, reduce the amount or shorten the leave-on time next session.