Once the water has cooled to a warm/lukewarm temperature, dunk the rag into the tea, and dab it onto the sunburn. DO NOT WIPE IT OFF! Leave it on and let it be absorbed into your skin. Once its dry, re-apply as necessary, but never wash it off. If you do this process before bedtime, you can go to sleep and give it time to work its magic. In the morning you can wash it off in your shower. The sunburn should be substancialy subsided or all gone, but remember: the sooner you can apply the tea after you get the burn, the quicker and more effective it will be.
Note: I had a severe sunburn and I also used the tea bags straight up, dabbing them directly onto my skin. You may use as many "coats" of tea as you want, but the more the better.