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 you...
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).