Always use body art quality henna from a trusted source. I recommend the following suppliers:
FYI: Never buy anything labeled as "black henna." There is no such thing in nature -- Products labeled black henna contain a nasty hair dye that gives bad rashes. See the warnings here.
The simplest henna mixes just combine lemon juice and henna powder. I prefer a more complex mix.
The night before, I simmered some black tea, cardamom, cloves, lime peels, sugar, and fenugreek for 30 minutes.
Next I mixed in key lime juice and let the mixture cool.
After the mixture was room temp, I mixed in 200 g Jamila henna and a few tablespoons of honey, adding enough liquid to make a thick paste. I covered it with plastic wrap and let it sit overnight. Here's what it looked like the next morning. You can see the top layer is dark because the dye release has begun.
Now that we've let it sit overnight, it's time for terps. If you want a darker stain, you can use essential oils that contain terpenes to help the dye release. For more info on terps, read here. For this mix I used a cajeput oil blend purchased from the Henna Caravan. I added 42 drops because I like that number.
Let the mixture sit another 8 hours or so, and it's ready to apply. I rolled cones out of mylar and filled them with henna, sealing the end with a rubber band. For other application techniques, you can read here.
Here's the henna applied when still wet:
Here's the finished result, before oxidation: