Here's what you need:
1) Embroidery hoop. For more stability you can use a canvas frame and a staple gun.
2) Sheer fabric. Tulle and Nylon don't work very well but you want something that you're going to be able to see through and paint will soak through.
3) Mod Podge
4) Fabric paint (whatever color you want)
5) Paint brushes
You'll also need a shirt or something to print on, and a pattern, drawing or design to print.
First you want to attach your fabric to your embroidery hoop or frame - make sure it is firmly secured and stretched out tight. Next you are going to trace your design onto the fabric.
After you've traced your design you are going to paint in all the NEGATIVE space with Mod Podge. This means you want to cover any area of your design that you don't want to be colored in when you print it.
Once the Mod Podge is is dry you put it down on top of whatever you want to screen print and gently dab the fabric paint (or screen-printing ink if you have it) through the screen.
Once it's dry you should have an awesome screen printed shirt (or bag, or something else) for cheap! Here's what I did: