Get a pot of water boiling. You'll probably want at least four cups worth of water to cover your tresses, but if you have super long, thick hair, just add more water.
Dump the hot water into a pitcher and throw in your bags of black tea. Use one tea bag for every cup of water. If you're using four cups of water, that means you need four teabags.
Let the teabags do their thing. Give the tea plenty of time to cool down. You definitely don't want to pour super hot tea over your head, so don't get impatient. When you think it's ready, check the temperature with your finger. If it's still hot, chill out for a while longer. Waiting only means the tea will steep more, making it extra potent.
Scrub up your hair with some shampoo while you're waiting on the tea to cool. If your locks are already clean enough, then just get your head wet.
Grab your cooled down tea and fish out the tea bags. Then put a bucket or large bowl in the tub or sink and hang your head over it.
Dump the tea over your locks and let it collect in the bucket. Now give your hair a little massage to work in the tea.
Pour the tea from the bucket back into your pitcher and repeat the whole shebang again. Pour, massage in, then repeat. To make sure your hair sucks up as much of the tea as possible, try to repeat the process about 15 times. If you start to feel crazy from hanging your head upside down, take a breather and then go again when you're ready.
Let the tea sit in your hair for at least 15 minutes once you're finished drowning your locks. Let it soak in longer if you want to go darker. When you're ready, rinse the tea out with water, then condition your hair.
If you don't have any black tea, you can use plain old black coffee to do this same trick. Just remember to leave out the cream and sugar.
If you have colored hair, try a little test strand first to make sure your hair's cool with the tea.