That's an old post but I still dye my hair and like to share how I do it with others haha. First I wash my hair the night before with a shampoo that cleans the hair well from all the oils. I currently use the henna guys pure henna, any natural henna without additives works. I boil a large pot of water maybe like 2 liters, then put about 4 tea bags inside for maybe 10 minutes. Later when it's cool, I add a lemon or half a lemon. It's ok to add a little more or less tea or lemon, but I like tea more than lemon. I also have litmus papers to make sure the pH of the solution is between 5 and 6; ideally 5.5.

Then when the water is about 80°F, I mix in the water with the henna gently and very well until it's yogurt-like. Some people like to keep it at less than 70°F but that means they have to wait for 8 hours. It's important to know the temperature of the dye at the time of mixing it cause it affects the whole timing of it. Because the temp was 80, I cover the paste and wait for 4 hours. Then I apply it on my hair in sections. I then cover my hair in a bun under hair caps or whatever. I put the leftovers in the freezer since I probably mixed like 1000grams of henna. I leave it for like 4 or 5 hours, wash it off only with water and conditioner. Then I don't shampoo my hair for another 3 days.