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When your hair is fully covered, take a long sheet of plastic wrap and wrap your hair. Wrap the plastic all the way around your hairline and completely cover your hair and the top of your head. Don’t cover your ears.  Wrapping your hair in plastic will help keep the henna warm and moist, and this will allow it to set.  If you have to go out while your hair is like this, you can wrap a scarf around the plastic wrap to cover it. Henna generally takes between two and four hours to set. The longer you leave it on, the deeper and more vibrant the color will be. You can encourage color development by keeping the henna warm. Stay inside if it’s cold out, or wear a hat if you must go out. You can leave the henna on for as long as six hours if you want to achieve maximum vibrancy. When the henna has had enough time to set, put your gloves back on and remove the plastic wrap. Hop in the shower and thoroughly rinse the henna paste from your hair. Rub conditioner into your hair to help loosen the paste. Continue conditioning and rinsing until the water runs clear and there's no paste left in your hair. Henna takes about 48 hours to develop properly. When your hair first dries, it will look very bright and orange. Over the next couple of days, the color will deepen and become less orange. Henna is a permanent dye, so you don’t have to worry about the color washing out or fading over time. You can reapply to achieve a deeper and more vibrant color, or just apply more paste to your roots as they grow out. When touching up roots, leave the henna on for the same amount of time as the original application to achieve a similar color.

Summary:
Wrap plastic wrap around your hair. Keep the henna warm and let it set. Rinse with conditioner. Wait a few days for the color to develop. Touch up roots as they grow out.