Mix the henna powder with an acidic liquid to release the dye. Most kits come with some type of catalyst --- put the prepackaged powder and liquid into a nonmetal bowl and mix with a plastic spoon.
Stir until all the lumps disappear and you have a thick paste. Add any special additives that come with the henna kit, such as sugar or essential oils. Keep stirring until the paste is smooth and has a consistency like yogurt.
Cover the container with plastic wrap, pressing down on the top until the plastic touches the paste. This will eliminate any air pockets. Alternatively, ladle your henna mixture into a plastic freezing bag and compress the air out before sealing it. Let the paste stand for 24 hours in a dark, warm place.
Transfer your henna paste into an included applicator with a plastic spoon or chopstick. Most likely, this will be a plastic cone. Set the cone into an empty jar or cup, and fill it two-thirds of the way with paste, squeezing downward. Twist the open end and seal it tightly with a rubber band.
Wash and dry the area for application, and then swab it with rubbing alcohol. If your kit has an application oil such as mehlabiya or mehndi oil, apply it evenly to the surface of the skin.
Slowly apply the henna design via stencil or transfer, without directly touching the skin. Use your pinky finger to support the weight of your hand, and correct any mistakes immediately with a toothpick.
Let the henna paste set for a minimum of 10 minutes, then apply a sealer. This can be either straight lemon juice by rubbing lemon slices on the design, or a lemon juice and sugar mixture. Allow it to dry for 12 to 24 hours, then lightly scrape the dried paste off with a butter knife. Freeze any leftover paste for future applications.