the process isn't terribly difficult for simple stuff, but it's a definite craft the more complex you get with your designs and colours.
assuming you can either operate photoshop, illustrator or corel, you print your design onto a transparency or vellum. note that i'm not offering and tips or tricks, just a barebones outline of the procedure. alternately, you can hand draw your design onto one of these. each colour requires a separate sheet.
you need to prepare a screen (a wood or aluminum frame with mesh stretched across it) by coating it with emulsion using an emulsion scoop and drying it (preferably in a drying box). emulsion is UV sensitive, so the darker the place you can do this the better, and you need to keep prepped screens in a dark storage area.
attach your design to the screen and put in the exposure unit, exposing it to UV light. once the image is 'burned' into the emulsion on the screen, it doesn't matter if you leave the screen out.
the screen is then rinsed. you can use a garden hose, but a power washer is preferred. this removed the emulsion residue where the image is burned into the emulsion, exposing the mesh so ink can be pushed through it. let the screen dry (i like to use an air compressor, but a fan or air drying is fine). inspect the screen for pin-hole sized defects by putting the screen on a light table (some exposure tables have a light table built in) or holding up against a light. (using a drying box helps keep dust and dirt off the screen, which will become small dots when exposed.)
repair holes or defects with tape or special 'paint.' repeat for every screen that contains a different colour, called a separation.
affix your screen into the press' clamp and tighten. spray some adhesive onto the platen and put a test piece (an old shirt or pre-cut felt pieces) on to the platen. (this process will vary from person to person.) put ink onto the screen, lower the frame onto the test piece, and use your squeegee to push or pull the ink over the design, forcing ink through the mesh and onto the shirt or felt. inspect for quality. cure, using either a flash dryer (that weird contraption on a stand next to the press set-ups you've probably seen) or heat gun (ideally, a final cure entails a conveyor dryer, but those are generally a luxury unless you're doing a lot of shirts).
depending on how you do your registration process, test you other screens on the test piece and align as necessary until all the colours in your design are where they need to be.
print your shirts. you'll hear advice that wet-on-wet (no flash cure between colours) is the way to go, or that you should 'flash' cure between every colour, or just flash your white undercoat if you have to have one for your design. i think it's going to depend on your printing and design requirements.
once you're done, the screens need to be cleaned. put the screens in a 'dip tank,' soaking them in a non-toxic chemical that 'loosens' the emulsion, then blast the emulsion off the screen with your hose or power washer. clean up.
of course, some people do things in a different order or use different methods to accomplish the same thing, and i make it sound harder than it *can* be (then again, it can be pretty damn difficult, lol) but that's essentially the way of it for a simple shirt using a typical one-station four screen manual press. this also says nothing about more advanced methods to achieve various results.
a lot of this equipment you can build yourself, like the press and exposure table, even a conveyor dryer if you're that handy. the one thing you can't build (unless you're some kind of wunderkind) is a flash unit and power washer.