Build a screen printing frame if you don't already have one. Make a simple, but sturdy, frame out of wood, slightly larger than the image you want to print.
Tightly stretch screen printing fabric over the frame. Put one staple in the right side of the fabric on the outer edge of the right side of the frame. Stretch the fabric across the frame and put one staple in the outer edge of the left side of the frame. Repeat this process with the top and bottom of the frame, trying to make the fabric smooth and taut everywhere.
Add more staples until you've stapled all the way around the frame, alternating sides as you go so that the fabric gets stretched as evenly as possible.
In a room with only red light, coat the stretched fabric with a thin, even layer of emulsion on both sides. There are special tools available for this, but a paint brush will work in a pinch. Allow to dry thoroughly without exposing to white light.
Draw or copy the image that you want to print on a piece of transparency, using black ink.
In the dark room, tape the transparency to your screen using clear tape. Expose the screen to bright light for an hour or so without jiggling the image.
Remove the transparency and wash your screen. The emulsion should wash off where your image was. If it doesn't come out easily, try spraying with a hose or rubbing gently (but be careful, because emulsion can come off in other parts of the screen too if you use a lot of force).
Dry your screen.
Put cardboard inside your t-shirt so the ink doesn't leak through to the back.
Flatten the t-shirt and position the screen over it, frame side up. Put a thick line of ink above the image and firmly pull the squeegee down from above the ink to the bottom of the screen. Repeat carefully if you want, but don't make more than a couple of passes, because the image can become smeared.
Carefully pick up the screen, holding the shirt down. Check the front for any stray blobs of ink before using it on another shirt, but if there are none you can go ahead and use it again. Keep printing, adding more ink as necessary.
If you want to reuse your screen, make sure to wash it well before the ink dries.