How do I make sure that your eyeliner and wing are not too thick?

Use a liquid liner or a gel (with a thin formula, like Tarte Amazonian Clay liner) with an angled brush, as opposed to pencil. You want to start at the outer corner of your eye and with one quick, fluid motion (it has to be fast or the wing won't be as sharp), flick your liner toward the tail of your brow to create the wing. Then, make a thin line from your inner corner to the tip of the wing and fill in the space between the line and your lash line. Take your time (apart from the flick) and practice a lot, because wings aren't easy until you get it right, then it's pretty easy.