If you're inexperienced with using Henna dyeing for hair, please make sure you do your research first or consult a professional :)
I used tattoo grade henna that has not been mixed with any ingredients. This is especially important if hair is already dyed.
The Henna powder will appear to be green, but will turn reddish-brown when you add water. I added enough water to get a paste-like or "pancake batter" consistency (this stuff gets MESSY and stains very easily. I wore an old t-shirt I didn't mind trashing, and mind your bathroom/sink/tub area). Sometimes I'll brew chamomile tea and add it instead of plain water, some people use lemon juice but I think it can be drying.
Mix it and let it sit (sometimes I let it sit in the fridge over night, or at least 4-5 hours)
Wear gloves and apply (work carefully)
Throw a processing cap or plastic bag over your head, let the Henna mix marinate for a couple of hours.
Rinse thoroughly! Rinse again! And rinse one more time! (remember, this stains very easily and is MESSY)
Condition/Moisturize hair with product of your choice.
Dry and/or style as you wish
My results above show my hair after a few Henna treatments, and after a trip to the salon for a wash, blowdry, and flat iron. My hair is a natural dark brown, but as you can see, it's picked up some of the deep red of the Henna. Also, my hair felt smooth, strong, and look at the gloss! This is a great natural alternative to chemical dyes, especially if you want this particular color. You can also mix Henna with Indigo to get a beautiful jet black. I haven't tried it yet because it involves a few extra cumbersome steps. Maybe I'll try it one day.
FIND HENNA AT AMAZON: Godrej Nupur Natural Henna