I made this last night, by tracing the image from my computer screen onto paper, then cutting it out using a craft knife. I then sellotaped it to my t-shirt, and used small pieces of double sided sticky tape to hold down any smaller details that may have otherwise moved.
I then applied the paint using a dabbing motion and a songe, this ensures that the lines of the image are crisp, as it prevents paint from getting beneath the stencil (as it often does if using a brush).
I then left it to dry and ironed the paint into the shirt (which I'mn told will make it more washable). It might be a better idea to use fabric paint though, I just didn't have any lying around.