I got this recipe from Hennaforhair.com
100 grams fresh Henna powder (I got mine from SFHerb.com)
500 ml. liquid (17 ounces). Half acidic/lemon juice, bottled works. The other half, use water that has been boiled.
2 grams powder pectin (0.8 oz.) Optional.
Mix the acidic juice (any citrus juice will work) and Henna powder. Note: The citrus is what helps the Henna bond and last on your hair.
Finished mixing all of the boiled water in.
Now to the bathroom. wear some disposable gloves because it is going to get messy. Please ignore the vitamins and prescription iron pills. ; )
Make soure you wear a towel around you or something that is not new. Or just strip down to your birthday suit or undies. Take a spatula and spread the mixture all over your hair.
I moved this to the bathtub. Very messy.
Now that you are ready to wash this stuff out. Get into a shower or bathtub and rinse as much of it out as you can.You will not be able to get it all out in this rinsing. wait 24 hour to wash your hair. As your hair dries you can brush more of the grainy parts out. So, brush your hair in/over the sink, shower or bathtub. It is more messy, but will clean up easy with wash cloth or vacuum.
Update: I find that since I have many grays and my hair can start to grow faster than I can make time to Henna, I have found a short cut. I have started to spray the front and top of my roots with Sun In. Let it bleach out my greys for a fews days or week, to blend and look a little better. Then when I have the time, I Henna my hair. I think the small amount of Peroxide in the Sun in is harmless and the extra ingredients are not bad enough for my concern of permanent issues. It is cheap and convenient.
In the end. I continue to do crazy things to my hair and stopped using the Sun in. Probably not the non toxic as I was wanting. But on the other hand I have read and tried a new way to rinse out the Henna. When you are ready to rinse the Henna out. Fill a bath tub up with water and get in. Put your hair in the water and swish it around. Then drain the tub and rinse with a large cup or in a shower. you can add conditioner to your hair and leave it in a few minutes and then rinse your hair in the tub or shower. This helps gets the a lot of the granules out.
I have not found adding conditioner right after doing my hair with Henna to cause a problem with color or lasting. I have not had a problem washing my hair to soon either and Yes, You can totaly leave the Henna on your hair for much longer. Like 2-4 hours or more. It is just about the time you have on hand. I have also used 2 year old/from purchased Henna and have it work just fine. Good luck.