From ancient times plants, fruits and flower extracts were used to colour hair, hands and nails. Henna was used on a large scale in India, Egypt and Middle East.

Here is a henna recipe
henna 2 cups
amla (gooseberry) 1 spoon
coffee powder 1 spoon
boiled and strained tea 1/2 cup made by boiling 1 spoon tea in 1/2 cup water
juice of one lemon (added just before applying henna to hair)
curd 1 spoon (added just before applying henna to hair)

Mix all these ingredients well (except lemon juice and curd) and let them soak for 2 hours. Now when you are going to apply henna to your hair, just a few minutes before add lemon juice and curd to the mixture.
Remember curd should not be added to henna when it is soaking for hours, curd should be added to henna just before applying henna to hair.

Apply the henna paste evenly to your scalp. Divide the hair into sections with a comb. Apply the paste to the roots and the hair section. Wind the hair section into a coil. Leave it for atleast half hour. Regular(once in 2 months) use of this mix makes the hair supple, shiny and keeps the scalp in good health.

Dont use metal bowls, spoons, hair clips when mixing or applying henna as they can react to the chemicals present in the henna.

Apply some cream or oil around your hair line, your ears and forehead. Also dont forget to use gloves or you will get red-orange hands. Wear an old t-shirt or black clothes, because henna stains clothes.

Some youtube videos for your reference

Before & After pics, henna
http://www.youtube.com/watch?v=kMeEw2LZnF0

henna for hair: half-gray to dark brown
http://www.youtube.com/watch?v=OObcuc6KpA4

HOW TO APPLY HENNA HAIR COLOR
http://www.youtube.com/watch?v=JqWHdgm6gRM

Henna Hair Dye (only use lemon for natural henna) use tea or coffee for dark colors
http://www.youtube.com/watch?v=7cJkX_e0cYY

henna tutorial for black hair #1
http://www.youtube.com/watch?v=8SIq5MokOpA

henna tutorial for black hair # 2
http://www.youtube.com/watch?v=gBiGwSYO5OY

henna tutorial for black hair #3
http://www.youtube.com/watch?v=zAWqnMJ_lDM

Mixing Henna for hair coloring
http://www.youtube.com/watch?v=eiYjDuOsHdg

Mixing Henna for hair coloring, part 2
http://www.youtube.com/watch?v=6P_JNosHQps

First Attempt at Henna
http://www.youtube.com/watch?v=iVSCBra7-8M