Acquire quality, pure henna powder from a reputable supplier. Avoid any product referred to as black henna because it can contain harmful additives. You want to use only 100 percent henna.
Measure your powdered henna into a glass bowl. Metal reacts with the properties of the dye, so use glass, plastic or ceramic. The amount of henna you use will depend on the intricacy and size of the body art, but 4 tbsp. should cover most applications.
Stir in freshly squeezed lemon juice 1 tbsp. at a time until the mixture reaches toothpaste-like consistency. The acid in the juice releases the dye. How much juice you need depends on the type, quality and amount of the henna.
Add approximately one part sugar to four parts henna. For example, if you use 4 tbsp. of henna powder, add 1 tbsp. of sugar to the paste.
Mix the ingredients until the paste is smooth. Add more lemon juice as necessary. Use a plastic spoon or rubber spatula to remove any lumps.
Cover the mehndi dye with plastic wrap. Let it rest for at least 12 hours and up to 24 hours to release the dye.
Adjust the consistency of the henna paste with more lemon juice, essential oils or floral water. The paste should resemble a thick yogurt or thin toothpaste.