It's pretty simple--just make tea in the regular way (maybe stronger than the way you would want it for drinking) and soak the fabric. The longer you soak or the stronger the tea, the deeper the color. You can experiment with scraps of muslin, tea of varying strengths, and the amount of time you leave it in (start with an hour and work up from there). When you like the color, dry it on high heat to set the "stain." Want it darker? Soak it again! :) We used to do this for projects in high school art class (plaster takes tea dye especially well).
If you just kind of dunk the fabric in and leave it, the color may end up mottled. If you want to be sure it is uniform across the fabric, stir it around a few times while it's soaking.