My friend and I are dressing up as Death Eaters for the movie premiere and book launch - we decided to complete the package by hennaing dark marks onto our left forearms.
Step 1: Template
Print off the following image and choose a size suitable for your arm. Cut out the design. Use an exacto-knife to cut out the eye sockets (this will give you a guideline for where to place the skull) as well as the 'holes' in the snake (where it makes the figure-8).
Step 2: Trace
Place the template on your friend's arm and trace around it using a pen. You could probably also use eyeliner for this.
Once the outline is complete, remove the stencil. Using it as a reference, fill in the details.
Step 3: Henna It Up
Start tracing the design using henna. I got mine off ebay. Try to apply the henna at a vertical 90 degree angle, this will help you be more precise and prevent your hand from smudging the work you have done before.
Step 4: Let It Dry
Let the henna dry. The longer you leave it on, the darker it's supposed to be. I left mine on overnight. I've also heard that steaming it (putting your hand over a kettle boiling) also makes the henna darker once it's set.
Once you've left it on long enough, scratch/rub the henna off. Wash off the pen/eyeliner with soap and water.
This is how mine turned out!