I used to be a henna head. Loooved it. Besides a gorgeous, unmatched colour, my hair was soft, silky, strong and had shine that would put a Pantene commercial to shame. Sadly, I just couldn't love being a redhead. It wasn't me, deep down. So, I tried to remove it. And destroyed my hair. And had to chop it all off.
A cautionary tale. Not because I don't thing henna is fabulous stuff. Because I do. Just be damned sure you are a readhead at heart, because once that stuff is set in your hair, it is bulletproof. It isn't going anywhere until you cut it off.
That said, henna isn't difficult or mysterious, at all. Order good, pure body art quality henna. (I used to get mine from henna4hair.) Do a strand test and let it oxidize for a few days so that you will know your colour result before you henna. Just collect shed hairs from the shower and your brush and comb and test the hairball, that way you don't have to cut a tress to test. Just mix it with hot tap water. Acid, like lemon juice isn't really necessary and can actually dry out your hair. So, skip it.
Leftover henna paste can be frozen for later use! Put it in a container, cover and toss it in your freezer (it won't stink it up or contaminate your food) and thaw it when you want to use it. Frozen paste actually gives more intense stain, as the freezing process breaks up the henna powder and really releases dye.
If you have any questions or want more info or hints and tricks for using henna, toss me a PM, I will be happy to talk henna with you.