Search online or in companion books for runes that are specific to the Shadowhunters universe. Since runes are the source of Shadowhunters’ powers, decide which powers you would personally like to have. Examples include Angelic Power, Clarity, Courage in Combat, and Mental Excellence. Since Shadowhunters often try to keep their runes hidden from sight, pick an area that would normally be covered by clothing. Even if you plan on hitting the streets without long sleeves, keep your character authentic and avoid areas like the back of your hand, neck, and face.  If possible, ask a friend to draw the runes on you, since this will allow you to cover more area with more runes than you could manage on your own. If you don't have anyone to assist you, stick to easy-to-reach places like your chest, stomach, arms, as well as the tops of your thighs. Depending on your flexibility, you could also possibly include more of your legs. For runes with more intricate designs, or even simple ones if you would rather not attempt drawing them freehanded, print each design from your computer. Use scissors to crop the paper until you have a rough square around the image. Then trace around the edges of the design, both inside and out, with a utility blade or exacto knife to create a cutout for stenciling. Tape the outside edges of the cutout to your skin and stencil yourself first with waterproof eyeliner, which will help prevent your rune from running should you start to sweat or otherwise get wet.  When you create your cutout, be sure to save any blank paper that has been cut from the inside of the design (such as the squarish piece in the center of the Angelic Power rune) so you can tape these pieces separately to your skin in order to complete the stencil. Do not use kohl pencils, since this material will rub off onto your clothes. If you don't have a printer, practice drawing your rune on paper a few times before you attempt to draw the rune directly onto your skin. Once you feel confident, tattoo yourself freehanded with black eyeshadow, since this is easy to wipe off your skin in case you slip up. Once you are happy with the result, trace over the eyeshadow with waterproof eyeliner for a longer lasting temporary tattoo that will keep its shape despite sweating. However, before applying any new products that you haven’t used before, apply a little bit to a small patch of skin to test whether or not you have any adverse reactions. If you are drawing your runes on your own body, be sure to practice drawing them according to your view of the area where it will be place. For example, if it will be on your chest, practice drawing the design upside-down since this matches your view of your own chest.

Summary:
Pick a design. Choose its placement. Use cutouts. Draw your tattoos freehanded.