We do this - part of our business -
The screen printing itself isn't that difficult, but if you're not doing it often, getting setup to print can be time consuming. You don't need a machine if you're using waterbased inks (air dry) instead of oil or chemical based (heat cure) inks.
Liquid emulsion can be used just as easy as the precut stuff.
I'm sure instructables has a few 'how to' articles on the subject - it's very similar to stenciling. You could, if you wanted to, make a stencil, and use that to print on a shirt until the stencil no longer holds up.
You can do lots of neat textures, appearances, etc with screen printing - glow in the dark ink, puff, raised, etc. Even glitter ink. Practice is really all you need. BUT, if you get materials from an art or paint shop, definitely look for practice squares. We buy our shirts in bulk, but it still sucks to mess one up and lose a couple of bucks!
You can even make your own screens easily. Instructables has a few articles on that too.
The big diff between pros and amatuers at this is the print quality, and volume. We turn out a few hundred shirts a week, not to mention the heat pressing, embroidery, and other materials. We do some jobs as low as 10 pcs., but I think we're bumping that up to 12.
Iron ons will work if you're in a pinch, but (personally) I don't like the quality. Sublimation is the way to go in that case, but a good screen printer can get very close to sublimation quality any day.
My .02....any other printers here? We also do ad specialties and a TON of vinyl.
Link to my site
here for anyone interested....