As far as I can remember, henna tattoos are pretty easy to apply.
You paint the skin in the design you want (a stencil works) with olive oil, with a small brush. You want just a thin film of the oil.
The henna powder is applied to the oil. Sprinkle it on, and allow the unmoistened henna to fall away.
Allow the henna to dry. It will fall away in clumps, or flakes.. the henna will have stained the skin underneath.
But do not take my word for it. There are a number of sites on the net which will help with henna.