Thanks to R.Balliot for the phrase, "there's a book for that". I painted two more shirts recently. I make them for friends. They're not for sale. I painted the first one for me over a year ago. I wear it occasionally on casual Fridays. It never fails to get comments from library patrons.
I use Neopaque acrylic paint by Jacquard and a stiff synthetic bristle brush. Masking tape helps me to keep the lines straight. After painting, the fabric air dries for a day. Ironing for 30 seconds front and back sets the paint. I've washed mine dozens of times and the paint is still fine.