We make stencil T-shirts all the time. There’s not really all that much technology involved.
To make stencils, we just use manila file folders. Draw on the folder, or tape on a printout off the computer. It takes a bit of an eye to turn, say, a color photograph into a recognizable stencil, so start with a simple design. Remember to include ribs for unsupported areas (think the inside of an “O”). Then cut it out with a nice sharp Xacto knife.
For paint, you can go a couple different routes. First, you can just use spray paint. Advantage is that it’s cheap and easy, disadvantage is that the color fades rapidly, so you get gray instead of black or whatever. Also, don’t use spray paint on 50/50 or synthetic fabrics. Something like 80% cotton is OK, but synthetics sort of… decompose a little.
The other route is acrylic paint, the stuff from the art supply store. Good, fast colors, but more expensive. Get a nice wide brush (say 1" or so), tape your stencil on the shirt on a flat surface, and go to town.
Does that cover it?