Summarize this article:

Make sure you put your gloves on first. The most crucial thing here is to coat your hair evenly with the henna mixture. Pay special attention to the ends and the roots, particularly along your hairline. Err on the side of using too much. When your hair is evenly coated, pile it on top of your head, and wrap your hair securely in a towel. Wipe off any excess henna with a wet washcloth. For best results, leave it in overnight; you may want to cover your pillow with a garbage bag, or something you don't mind getting dirty. If you don't want to sleep with the dye in your hair, you can simply leave it in for a few hours. The longer you leave it in, however, the more intense the effect will be. The greater change you want to effect, the longer you will need to leave the dye in. It is easier to darken light hair than it is to lighten dark hair. If you have very dark hair to begin with, even leaving henna in overnight won't make you a strawberry blond. You will want to wear gloves for this as well, or your hands will be stained orange. Be very careful; it is easy to dye things you don't want dyed. Depending on the length of your hair, this part of the process can take anywhere from 5 minutes to an hour. Kneel over the bathtub rather than standing in it, or you will dye your whole body. Carefully remove the wrap covering your hair. Rinse thoroughly, until the water runs clear. Get into the shower. Apply shampoo, and rinse. Apply a deep conditioner and let it sit for 10 or 15 minutes before rinsing. Go check out your new hair color in a mirror! Don't wash it or get it wet for another 24 - 48 hours.

Summary:
Work the henna through your hair. Let it sit. Rinse out the henna. Let your hair air-dry.