There's actual paper that you can print the design onto then you can iron it onto a shirt. You should be able to do that pretty simply.
how long does that last on shirts?...
They usually last quite a while. Infact, I'd say most of the shirts people buy at concerts are like that.
I've got a shirt that I made that says "Billy Idol Started Punk" on it, and the shirt's falling apart, not the decal. (I've had it a few years, now.) It just depends on how often you wear it, or strech it, or wash it, and other things like that.
Plus, it's pretty easy, you just iron on the paper side, and then the image will transfer. But you'll need to remember it'll be a mirror image, if it's text.