Buy triple sifted henna such as Jamila or Rajistani
Gather your ingredients: henna powder, bottled lemon juice, Sweet and Low (or sugar, honey, or molasses if you don’t have Sweet and Low), essential oils (lavender, cardamom, cadjeput, and tea tree are all great), a plastic spoon, and a plastic or glass container such as a small yogurt tub (something you don’t mind getting stained). Metal spoons and dishes will make it so that the henna stain is less powerful.
Mix one part Sweet and Low (or above substitue) to three parts henna. Next, add as much lemon juice as you need for henna to be the consistency of frosting or toothpaste (adding in small amounts). Err on the side of drier henna rather than too-wet henna. Henna will be a bit lumpy.
Put lid on henna mixture and allow to sit in a lukewarm place (counter or top of fridge) for a minimum of 24-36 hours before using for best results. If you live in a cold climate, more time may be needed. If you live in a very warm climate, less time may be needed.
Once 24 hours have elapsed, it’s time to add the oil blend! For a small batch, mix 10-15 drops of essential oils for each two spoonfuls of henna powder. For larger batches, such as 1/4 cup henna, mix with 1-2 teaspoons oil.
Mix well with a plastic spoon, add lemon juice (if needed) to get desired consistency, and spoon henna into a cone. Henna will be darker in color on the top and bright green underneath before mixing- this means the dye is releasing (a good thing)!
This henna can be used for the next three days at room temperature. After 3 days, the henna expires and does not stain very well.
As an alternative, you can freeze your henna after you have waited the initial 24-36 hours. This henna, if kept frozen, will keep for about a year, although staining power will fade with time.
I have high quality kits and bulk henna available for sale if you are interested in trying henna for yourself!