Just as a basic run-down to go from art to a shirt with a screen press, from scratch:
Build or buy a screen ($30-$50, or ~$10-20 + an hour)
Print 2 transparencies (75 cents each at kinkos)
Coat and expose the screen (I think it’s $5 donation per screen, unless you buy your own for $16-$25, hour to an hour and a half)
Wash out the screen (15 mins maybe)
Pull the prints (10 mins for practice on a test shirt, another 10 for the pulls. Bring extra, it’s not guaranteed to be a good pull, especially if it’s multi color)
Clean up the screen (10-30 mins, depending on the ink)
Recover the screen to remove the stencil (30-45 maybe, much faster per screen if you do in bulk)
I’m all for teaching a man to fish, but that’s definitely the long way around for 2 shirts.
Edit: I take most of that back. If the design isn’t too complicated, you can cut it out of vinyl and use the other side as a stencil and pull a few prints. That would get rid of a lot of the time and hassle.