I bumped the huge Henna Thread 
I've been henna-ing since Easter , before I was using a Superdrug dye and highlights (myself) my hair is short , cut every 5-6 weeks and I was box dyed every 3 weeks.
I started as brown and went pretty light coloured.....then I got bored , my hair became very unhappy with box dye so I went out on Easter saturday , had it scalped and hennaed that night .
My Before/After photo is dated 28 March.
I started brun then brun/noire , increasing the noire to 100%
Gradually cropping out the dye now my hair is soft, thick. I don't get the shine that other users get , but I'm 50 so my hair is menopausal with a good % of grey.
It Does intensify with use, I was a bit meh when I used the noire, it went red setter brown but now (I haven't done henna for a while, about 7 weeks) its really dark, Bourneville Chocolate brown. 
Heat intensifies red but it needs to be warm to melt it and I really advice lots of clingfilm to contain it. Put some thick cream round your hairline and some cotton wool or tissue under the towel. I use clingfilm and a black hair wrap/turban.
Melt in a pyrex bowl over a pan of boiling water. I don't grate or chop mine, I only use 2 blocks. I keep stirring it with a plastic spoon (no metal with henna) and let it 'cook' for 15-20 minutes.
You want a thin, single cream texture.
I put a huge plastic bowl in the bath, lean my head in, apply (DD points out the missed bits) wear gloves .
Wrap and potter about, I do 4 hours.
I wash in the shower , use cheapie conditioner to remove it , then shampoo, then rinse the shower and put the curtain in the wash.
I love Lush Henna, my hair has improved 1000 times , but it is a faff and like all dyes , a commitment.
Its not as strong as regular henna, more a glaze but it does coat the hair.
It changes over the 48 hours after applying, the colour intensifies.
Always do a strand test. Then no
surprises.
Good Luck.