Pour a box of laboratory-tested, body-art quality henna powder with at least a 2.3 percent lawsonia content into a plastic mixing bowl. The higher the lawsonia content, the darker red your gray hair will be.
Add 2 tsp. of apple cider vinegar and 1 tbsp. of olive oil per box of henna. Olive oil makes application easier and provides a deep conditioning treatment while you dye, while vinegar helps the color stick in gray hair.
Boil 2 cups of water in a saucepan on the stove or in a bowl in the microwave. Use a darker liquid rather than water to provide a darker shade, as henna has a tendency to be bright orange on gray hair. For burgundy, substitute 2 cups of red wine. For a dark auburn, use 2 cups of strong black coffee.
Pour the boiling liquid over the henna, stirring constantly with a plastic or wooden spoon, until the paste is the consistency of creamy soup. You want it to look like mud but not be so thick you can't glop it through your lovely gray locks.
Let the henna paste cool enough that it's comfortable to touch, then put on latex gloves and start piling it onto your hair. Don't expect there to be anything neat about this process. It's super messy, so you have to embrace that as part of the fun. Put on a plastic shower cap to contain the glop and your body heat, which helps the color take.
Leave the paste on your hair for as long as possible. The longer you leave it on, the darker the result will be. When it finally starts to dry and crack, get into the shower and rinse it off. For added enjoyment, pretend you're in a landslide -- that's what your shower will look like when you're done! Prepare for some serious bathroom scrubbing after dyeing with henna.
Shampoo your hair with a moisturizing shampoo to remove all the leftover mud, then condition if necessary and style as usual.
Freshly hennaed gray hair may be Bozo the Clown orange at first, but don't let that alarm you. The color will mellow and darken over the next few days. The good thing about henna is that it's actually very conditioning, so if you want to repeat the process in a couple of days to achieve a darker color, go for it. Your hair can only become more fabulous!