So I used cardboard for the first two. It was tough to cut without a boxcutter, but i used scissors and the edges don't need to be perfect so that is fine. I JUST finished the LoL shirt (did a trinity force as suggested by ThaMortal) which had a much more complex shape, so i printed out the icon (its low res when you blow it up) and taped it onto a piece of heavy paper. I used heavy duty construction paper, but you could probably use that heavy printer paper or photo paper. The end result is now in the wash, but it looked really good.

As for stencils, I just found silhouettes, its much easier.

Make sure you use 50% bleach/ 50% water, put a piece of cardboard inside the shirt to prevent leak through, spray over the stencil, let it sit for a while until it starts to discolor, then dump it under running water (bath or sink) to stop the process. Rinse it a bit, then toss in the wash.

Edit: That teemo stencil is great, but might be tough to do properly because of all the individual parts (you'd have to eyeball the alignment.) It DEFINITELY would look stellar on a shirt though. Good luck.