The first key factor is freshness of your henna. It's hard to tell just from the color. I have seen bright green give an awful stain while a brownish-olive green gave a fabulous color. Buy from a reputable dealer is the best suggestion here. With fresh henna, the better, darker, longer lasting stain you will get. It can keep in your freezer in a airtight zip lock (in a brown bag or aluminum foil) for up to a year.
1... If you have in sifted henna, the first thing is to sift! This is very important so you won't get tiny fibers clogging your cone (or bottle). Do this 2-3 times.
2... In a ceramic bowl place about 2 tablespoons henna powder. Add enough lemon juice to make a thick paste (anything acidic will work, lemon, lime, tamarind, etc.) Let this sit over night covered with plastic wrap. (The colder it is the longer it will have to sit. at 75 degrees about 12-15 hours, 95 degrees as little as 1 hours) HINT: If you have a gas stove with a pilot light its about 95 in there and will only take about 1-2 hours.
3... You can tell when its done by the surface. It will have a brown appearance and there will be brown liquid in places. You can also let it sit in a zip lock with a paper towel under it and when its done it will stain the towel. The henna tonic dye is small enough to pass through the zip lock (This hint is Catherine Cartwright Jones Idea) Add coffee or tea to get it the consistency of toothpaste. (Also you can use lemon, eucalyptus oil, cloves etc) You can strain it again at this point. This trick really works great! (Maison Kenzi came up with this idea) Take panty hose, or a piece of cheesecloth and fill with a few tablespoons of henna paste. Place inside a zip lock bag and squeeze and twist until filtered through the cloth. Squeeze the paste down to corner of the zip lock bag and snip the corner for easy no mess transfer to a bottle or bag to apply.
4... So its ready. now what? You can use a number of things to apply henna with. A cone, carrot bag, syringes, jacquard bottles. I personally use the carrot bag or a cone. Put your henna in your bag and your ready. Choose a design and henna it. You can use a toothpick to fix small boo-boo's you might get.
5... When your done or as you go depending on how fast it dries you can do a few different things. (these are a matter of personal style and like ) You can choose to let it dry and flake off on it's own, or apply lemon and sugar mix (2 tsp lemon, 1 tsp sugar, 2tsp water) once it it fairly dry (so you don't smash the design) to keep it on and moist. You will need to reapply when it looks like its starting to dry. Liquid latex, and new skin are also acceptable. (advice: don't put latex where there is hair OUCH! Don't use if you have an allergy to it)
6... You can choose to wrap it now to help it stay on and get a beautiful dark stain. It helps the henna stay in place and keeps the skin moist and warm. Wrap the area with toilet paper firmly but not to tight, careful not to dislodge your henna. Wrap this with saran wrap and tape. (should be like as firm as an ace bandage.) Sleep in it.. You should leave henna on as long as possible. Min of 3 hours.
7... When your ready to take it off cut off the wraps. You can steam it to help loosen the henna (also helps make darker) and scrape it off or use a little oil (eucalyptus or olive) to wipe it off.
8... Do not get wet for 12 hours at least. It will not darken properly if you do. After that you can help staying time by using the oil rubbed onto it before you bathe to repel the water. DO NOT use Vaseline, mineral oils. These will fade the stain. Other things that will fade it are bleach, detergents etc.. Different areas take the stain better ..hands and feet are excellent while arms, back, chest areas don't stain as well. They can last 1-3 weeks (sometimes a little longer) depending on the care you take of them.