I'm a little late with this post,but I did my first henna treatment about 3 weeks ago. To be honest I was doing it to cover the few strands of gray hair that I had peaking through my fro.
I co-washed first with Aussie Moist (I LOVE this stuff) then i decided to also wash my hair after the co-wash. A lot of people only co-wash before doing a henna treatment but I thought it would be better to have clean hair so that there was no barrier between my hair and the henna. I washed my hair with this shampoo bar I purchased from an Indian store(i'll do a seperate post about this shampoo bar later).
After shampooing I applied the henna. I purchased my henna from Mehandi they make a finely sifted henna just for "African Hair." I mixed my henna with lemon juice until it became the consistency of lumpy mashed potatoes, covered it and let it sit over night. The next day before applying my henna I added 2 tablespoons of raw honey and brewed a cup of green tea which i added to the henna until it became the consistency of yogurt(next time I will go for the consistency of cake frosting). I applied the henna to my hair and covered it in a plastic shower cap and went to bed.
The next morning I filled my bath tub with water and bent over the tub dunking my head in the water to loosen the henna until most of it was out. Then i hopped in the shower and rinsed the rest of the henna out. I applied Aubrey Organics Honey Suckle Rose Conditioner to my hair and covered with a plastic cap for an hour to achieve a good deep conditioning. And that was it...i rinsed it out and my gray was covered perfectly. I plan on doing henna every month.
By the way, I used the henna with the auburn stain, and it only took 100g to cover my hair.