In early days, mehndi is made of freshly ground henna leaves & tamarind. Now, the henna leaves are dried & powdered, which is mixed with henna oil, lemon juice, tea & tamarind. This is used to apply mehndi designs. You can purchase henna powder from most of the Indian stores.
Using a tea strainer sift the henna powder. Add 5 drops of eucalyptus oil (or) mehndi oil, strained tamarind paste, 2 pinches of sugar & tea (or) water to the henna powder. Mix this into a fine paste. Keep the paste covered for atleast 2 or 3 hrs for the dye to develop.
Take a rectangular plastic sheet, leave 1 inch at the end of the long side & roll it to a cone with a small hole at the tip. Stick the side of the cone using a piece of adhesive tape so that the cone holds its shape. Then fill 3/4th of the cone with the above said mehndi paste. Roll the top the cone & cover it using adhesive tape. Now the mehndi cone is ready to use. Instead of making this cone you can also buy applicator bottles or plastic applicator tubes with different sized tips, which is available in some Mehndi Stores.
Using your creative mind either you can create your own designs by squeezing the tip of the cone or you can buy some design books & stencils which are available in the mehndi parlors. Then try simple designs first and by practice you can put complicated patterns.
Mix 3 tablespoons of lemon juice & 1/4 teaspoon of sugar & 1 drop of eucalyptus oil. Mix this well until the sugar dissolves.
Apply this solution on the mehndi while it is drying (dabbing with cotton) so that tha mehndi remains wet & absorbs more out of the mehndi. Repeat this whenever necessary (atleast 3 times). Let the mehndi be in your hands for atleast 3 hrs. Longer it stays on your hands, brighter the color appears (more than 5 hrs).
Then scrape off the mehndi and apply lotion on the hands to remove the stickiness of the lemon-sugar soln.
IMPORTANT: After removing the mehndi DO NOT WASH YOUR HANDS for atleast another 12 hrs because the mehndi stain on the hand will be very light orange in color, afteryou remove it. But it will get darker over the next 24 hrs.
Other Mehndi link:
She has a very nice page with full of mehndi related informations like mehndi tutorial, art gallery, artist locations, Henna suppliers...