Henna or mehndi is not a tattoo which is made with a needle to bring the color pigments beneath the upper layer of the skin and therefore painful and permanent.
To draw such fine lines the finely grounded laeves of the henna shrub is in a first step well sifted, than mixed with warm water. After several hours of let sit the paste the strained paste is filled in a cone or syringue to lay very thin and delicate strings on the skin, which will stain the skin red. THlS DOESN'T HURT AT ALL, but you have to be careful not to smudge the henna and also very patient, because henna needs between 6 to 8 hours to develop a color.
This picuture shows the mehndi after painting the henna on the hand. For that the henna rest moistened for several hours it is sealed with a coat of sugar water.
On my website www.henna-und-mehr.de (available in French and German) you can see several little videos
how henna is mixed and applied:
1.) how the paste is prepared: www.henna-und-mehr.de/movies/hzuberei_T1.mov
2,) how henna is applied with a syringe: www.henna-und-mehr.de/movies/hennaauftrag_T1.mov
3.) how it is applied with a cone : www.henna-und-mehr.de/movies/hennaqidra2.mov