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Tear off a strip of tape approximately 1–2 inches (2.5–5.1 cm) in length and press it into place over the line created by the overlapping material. Run your finger back and forth over the tape a few times to make sure it stays put.  Any variety of tape could work, but clear tape tends to be the most forgiving to work with, since it tears and smooths easily and can be peeled off and reapplied if you happen to make a mistake. If you like, you can also cover the seam on the inside of the cone with a second strip of tape to be doubly certain that it will hold. Once you’ve mixed your henna paste, use a spoon to scoop it into the open end and shake the cone gently to help it settle. Be careful not to overfill your cone. Doing so will make it harder to seal, and could result in an unexpected mess. Henna stains very easily, so be sure to pull on a pair of rubber gloves before you begin handling your homemade henna paste. It may also be a good idea to cover your work surface with a layer of paper towels or newsprint.

Summary:
Place a strip of tape along the seam on the outside of the cone. Fill your cone about 1/2 or 2/3 of the way up with henna paste.