Put Scoby and finished Kombucha in a gallon jar. Cover with a paper towel or cheese cloth and secure it with a rubber band.
In a 3-quart pot over medium heat, combine tea bags, sugar and 3 cups water. Bring to a boil, and then remove from heat and cover. Let steep for 15 minutes, covered.
Squeeze liquid out of tea bags, and then remove. Add 2 quarts of cool water to the pot.
Add cooled-down tea to the gallon jar. Add more water to fill to shoulder of jar, just before jar starts narrowing in diameter.
Cover with paper towel or cheese cloth and secure with a rubber band. Leave at room temperature for 5-10 days, until it's bubbly and tastes slightly sweet and a little sour.
Pour finished Kombucha into another gallon container, leaving Scoby and 1 1/2 cups of Kombucha behind. Repeat the above steps to make more batches.
Transfer finished Kombucha to bottles or tightly lidded jars. Leave at room temperature for 1-2 days to build up carbonation, and then chill until needed. Finished Kombucha keeps indefinitely, but gets more sour and vinegary over time.
Variation: For a darker, richer Kombucha (which is more traditional), use black tea instead of green tea.
Second Ferments: This would be after your first ferment is finished, you can add any number of combinations of fruit, herbs, and tea blends, etc. to add a different flavor profile to your Kombucha.
I sometimes add Blueberries and a piece of Rosemary to the jar and let it sit for a couple of days. I also use some nice medicinal tea blends. Raisins add carbonation too. It only takes a few per jar. I also put pieces of fresh ginger in too.
Something I usually recommend is to taste the Kombucha when you first are making it, that way you know how sweet it is.
After about 5 days, taste it. You're looking for a slightly sweet, slightly sour taste. If it's still too sweet, let it continue fermenting.
You'll get the best carbonation if you can get it to the above state. If it gets more vinegary, it's still okay, your carbonation won't be as high and the taste will just be slightly different, which is okay. Some people like it this way.