Henna is sold as a powder to be mixed with water and in tubes as a premade paste. The henna used for mehndi, or temporary tattoos, will be brown when applied to skin. You can buy henna at Indian and Middle Eastern grocery stores, but you'll want to ask how long it's been on the shelf, as it loses its dye strength over time. Remember that black henna is a toxic coal tar dye that can cause serious skin irritation and should never be used for temporary tattoos.
Mixing your henna yourself usually gives the best results. All you need are henna powder and water, but some artists add vegetable glycerin, coffee, or essential oils. Experiment with your mixture until you find one that produces the ease of use and depth of color you prefer.
The most common tool for applying henna to the skin is an open-tipped cone. It's easy to create your own henna cone using a sheet of Mylar tissue or a plastic freezer bag. Thicker Mylar is easier to work with and makes a sturdier cone. Roll the sheet or snip the end of the freezer bag to create the size of hole you want at the tip. Fill the cone with henna, twist the wide end, and squeeze the paste onto the skin. Some artists prefer to apply henna with a toothpick as it's more like drawing with a pen.
For those who would rather not create their own designs, there are a wide variety of pre-made stencils available. The Indian or Middle Eastern grocery where you buy your henna will often sell stencils as well. Hand-shaped templates are available if you want to go the traditional route, or you can choose a design that looks more like the ones on the walls of tattoo shops.