This is the tutorial I made for the Craftster Challenge. Thought I'd share it over here.
So years ago I fell in love with henna and started to do my own. With quite a bit of practice I got to be pretty good. It's been a while since I've done it, but with all the lovely henna designs on our Bath and Beauty board I thought I'd give it another go.
In this tutorial I will show you how to mix up your own paste and apply it.
The first step is to procure your henna. In this tut I'm going to show you how to mix up your own from powder, but it can also be bought pre-mixed in a variety of forms. My favorite pre-mixed form to get comes in a metal tube (sort of like an ointment might) with a screw on plastic tip. However, I've only seen this once and made the mistake of only buying one tube. The most common way you'll find henna paste is in a cone.
Why do I mix mine from powder? There are two reasons. First, I find it easier to get. This box I bought at The House of Spice in Kensington Market, Toronto. I have also found it in health food stores. The second reason is then I know what's in it. The powder itself should be pure ground henna leaves.
Note: Do not use hair dye henna on your skin. Also avoid anything claiming to be black henna. They quite often mix in some nasty chemicals which are not good for your skin.
To start you'll need your powder and a lemon or two. My lemons were big, so I only needed one.

Take two heaping tablespoons of powder and put it in a container or bowl. If your henna powder contains sticks or lumps you will need to sift it first. We want a nice fine powder, like baby powder. To sift simply stretch a nylon over your bowl and sift one spoonful at a time through.
Your powder should look like this. (That's right, it's green. Also sorry for the out of focus pic.)

Next, squeeze in your lemon juice. Avoid getting any pips in there. Remember we're going to want a nice smooth paste at the end. It's not a bad idea to put the juice through a strainer. (I never do.) Mix it up, adding enough juice so you have the consistency of instant mashed potatoes.

Now let it set overnight covered loosely. At this point I like to brew up some nice strong tea by putting 2-3 tea bags in my tea pot and filling half way with hot water, then just letting it sit overnight. We'll need this for the next step. You can also use coffee or espresso in place of tea.
The next morning/day you'll find your paste has darkened. Grab it and your tea or coffee. You can also use water in this step.

Stir in a little of your chosen liquid at a time, mixing well. We're going for a smooth paste slightly thinner than tooth paste. If you take a big glop of it on a spoon, it should fall off in 10-15 seconds.

Now let it sit again for a few hours. (Isn't this waiting getting annoying. It's not 100% necessary, but it helps give your henna a darker, richer colour.)
There are several ways to apply henna. I usually go for what seems to be the most common, which is using a cone. This works sort of like an piping bag. In fact you can use a small, disposable one if you have one handy. The key is to have a very fine tip.
Other ways to apply henna include using a tooth pick or fine brush. This may be easier if you're not used to a cone or piping bag. You can also tape off designs with masking tape, applying the henna over the whole surface, and removing to tape. The parts covered by the tape will remain skin coloured. This is usually used on the bottoms of feet to get a mover geometric sort of design.
To make a cone you'll need some sort of tape. Here I'm using masking tape cause it was handy. You can use almost anything. You'll also need something to make the cone with. For this you can use one half of a freezer bag, waxed paper, or freezer paper. You need something that is somewhat waterproof. I'm actually using a page from a paper palette. It's pretty much just freezer paper, about 8"x10".

Roll your paper into a long narrow cone and secure it with the tape.

Make sure you have a teeny tiny point. You can make a small one bigger, but you can't make a big one smaller.

Secure with loads of tape. Make sure every seam is covered or you henna will seep out. This is my finished cone beside a chop stick for size reference. I've cut off the top bit to make it shorter.

Fill with henna paste. Make sure to leave room to fold down the top.

Fold down the top and secure with loads more tape. (You can't use too much tape here. It will try to seep out everywhere.)

Now we need to get this cone going. This can be tricky. Squeeze and massage down your cone until the henna starts coming out the tip. You may need to cut your tip bigger, but be careful not to make it too big. Just snip a tiny bit at a time. I like to use a pin to help getting things going. Stick it in the hole and sort of stir it around. This helps slightly widen the hole and break up any lumps that might be clogging it. If the henna doesn't seem to be coming out just keep trying to work the henna down into the tip.
Do this over paper towel because it will all of a sudden squirt out everywhere. You may also find small leaks in your cone, just tape over them with lots of tape. (Tape is good.)
This is my mess trying to get my henna going.

Once your henna is flowing you can start designing. Practicing on paper first is a good idea, or just use somewhere on your body people won't really see. It also tends to be easier to design on someone else, but that might not be a good idea until you get the hang of it. If you get henna where you don't want it, wash it off immediately. It starts to stain within a few minutes.
The darkness and colour of your design will show up differently depending on your skin type. For me designs only show well on my hands (palms in particular) and feet. Any where else and I only get a faint stain that does not last. It's also a good idea to give your skin a good scrub with soapy water before applying henna the help the design stain well.
Keep the paste on your skin for as long as possible to get the best colouring. Some people like to re-wet the paste as it's drying with lemon juice. Just dab a little juice on the drying paste with paper towel or cotton ball. The paste will naturally dry and crumble off, but some will stay stuck to you. For best results do not use water to remove these stuck bits. Water is your henna design's worst enemy. Instead use oil, any type will do. (Literally. I've used corn oil.) The colour may/should darken over the first couple of days.
Here's one I did on my hand with the paste still on.

And here it is with the paste removed.

For design inspirations there are various books and websites. Or just go for it and see what happens. Getting used to the cone can be tricky, but with a little practice anyone can do it. I usually use a pin to help with smaller details.
To extend the life of your design avoid washing it. When it's shower time, cover your design in a thin layer of oil to protect it if possible. Vaseline works well for this, but even just some body lotion will work. If your design is on your hand like mine, you probably shouldn't avoid washing it. Hand washing is important people.
To store your cone stick you pin in the end to stop it clogging and keep in the refrigerator. (Keeping it in the fridge may not be totally necessary, but you don't want to risk your henna going bad. It is made with plant material.)
Have fun with you henna!!