Use plastic bowls and a spoon when mixing your henna.
Sift the dry henna through some cheese cloth first to make sure there are no lumps or tiny twigs in it.
Boil 1 cup of water, a few cloves, a few black tea bags until the liquid is reduced to about 1/3 of a cup.
Squeeze a lemon and strain juice. Add the strong tea and then the lemon juice, one tablespoon at a time until the henna is the consistency of thin toothpaste. Be sure to add a pinch of sugar to the henna after you add the liquid.
The henna should sit overnight before putting it into the cones.
For more info and pictures: The Henna Page Mixing Instructions