The title is imperative for a reason. Seriously, you should either be working on a t-shirt right now or headed out to Michaels to pick up supplies. Oh, or reading this post. That's totally fine.
To make this Princess Peach shirt, HackerZen user Lenore adapted the technique used by a very faithful reader to make his Phoenix Wright shirt, adding the extra step of drawing a grid on the shirt with a washable marker, then filling in pixels to make a big sprite. Excellent.
But don't just default to Mario. There are a lot of sprites out there. Get creative!