Boil the water and steep the tea bags for 5-7 minutes. Remove the tea bags and dissolve the sugar in the hot tea. Allow the tea to cool to room temperature. Transfer the cooled tea to a large glass jar and add the SCOBY and starter kombucha. Cover the jar with a breathable cloth and secure it with a rubber band. Let the kombucha ferment at room temperature for 7-14 days. Taste the kombucha to check for desired tartness. Bottle the kombucha, adding optional flavorings if desired, and let it carbonate for 2-7 days before refrigerating.