Yes, I used to use this a lot. If you end up liking the results, start taking the free shower caps from hotels because they're a lot easier for containing this slop than anything else.
I used to surround myself with newspapers and sort of sit amongst them because it would fall off and stains things quite easily.
You put it in a bowl and pour boiling water on it - say, about enough to cover it, but not too much because you can add more later. Leave it for a bit (minute or two), and it starts to go soft and then you can stir it into a paste. I used to stir it with a chopstick. It smells very grassy and organic, a bit like horse poo.
Then you slap it on your hair, wearing plastic gloves, and sit there for as long as you think is a good time. I found anything under an hour was a waste of time, but I have fine untreated hair, apparently it works a lot more quickly on hair that's been coloured or whatever before and thicker hair that takes colour better than fine.
Then you go and rinse for about 6 years. And shampoo or else you'll never get it out! Here's their online cartoon.
www.lush.co.uk/categories/Henna_44.aspx