If you have chemically dyed hair, you have to do a pretest because sometimes henna reacts strangely to chemical dyes. While I admit that I didn't do the pretest, I can't stress how important it is for the average person. So do it! :-)
The other really important thing to remember is not to use any metal when you're preparing your henna--this includes any brushes or spoons you use as well as the bowl you heat your water in. I recommend heating your henna in the microwave in a glass bowl. Do not put your glass bowl on the cooktop because it will explode. (I learned this the hard way.)
On to the henna. I used Light Mountain henna for gray hair. It comes in lots of shades--from light golden brown to red to black--and is a two part process. The first step is a 100% henna and the second is a blend that provides the non-red pigment.
Step One: Boil bottled water in glass bowl (in the microwave!!!) and put vaseline along hairline and over wrists.
Step Two: Add water to first bag of powder with one egg and mix well with wooden spoon--the egg was recommended by a woman at work to make the mixture "sticker" and less drippy, it's also good for your hair.
Step Three: Gloop it on in no particular fashion--it's green and smells like strong herbal tea. This step should last 5 to 15 mins but I waited 30 because I know my gray is extra "stubborn"
Step Four: I knelt next to the tub and rinsed my hair using the shower sprayer. At this point the gray (and my scalp) were orange and I was mildly alarmed.
Step Five: Boil more water and add it to second bag with another egg. Again, you end up with a lovely green mixture.
Step Six: Gloop the second mixture on and cover it with the plastic bag provided. Again, I left it on way longer than they recommend, about an hour.
Step Seven: Rinse again in the tub and once the water runs clear, rinse with diluted Apple Cider Vinegar--a 50/50 solution.
The nice thing about henna is that I can reapply it as often as I want. It's semi-permanent, which means it will gradually wash out, although I'm hoping that since I don't use detergent shampoos it will last pretty long, but I'm planning on doing it every 4-6 weeks. It's left my hair soft and silky and I'm really, really happy. Can you tell?