If your initial batch of plain kombucha still has the SCOBY in it, remove it along with 2 cups of the kombucha tea, and place it in a glass jar. Cover the top with a breathable cloth, like a cheesecloth or thin tea towel, and secure it with a rubber band. Store it in a cool and dark location, like a pantry or cabinet. This is what you'll use to brew a future batch of regular kombucha.
Warm 1 cup of water and dissolve 1 tablespoon of the sugar into it. Let the water cool to just slightly warmer than room temperature, then stir in 1 teaspoon of yeast. Let it sit for approximately 30 minutes to activate (it's active when it's foamy with a few bubbles).
While you wait for the yeast to activate, pour the rest of the sugar into the kombucha tea. Stir for a minute or two to dissolve the sugar.
Pour the kombucha into your gallon glass jug, then mix in the yeast slurry (if you are using two half gallon jars, give the slurry a quick stir, then pour half of it into each jar).
Follow the instructions for your specific airlocks, then close the lids on the jars. Leave them out to ferment in a cool location away from direct sunlight. After a few hours to a couple of days you should start seeing bubbles forming inside the jars and in your airlocks.
Let them ferment for about 7-14 days, or until the activity in the airlock slows down. The total fermentation time mainly depends on the temperature in your home. The cooler it is, the slower it ferments; the warmer it is, the faster it ferments.
When you no longer see bubbles or pressure in the airlock, remove the lid and taste your hard kombucha. It should taste like regular kombucha, but on the drier side and with a slight champagne flavor.
Whether you choose to stop here, or move on to adding flavors and carbonation, you'll need to first remove any yeast build-up or pellicles that may have formed on the surface of your jars, but do not mix the hard kombucha at this point.
Transfer your finished kombucha into another jar or smaller bottles using a funnel. Do this carefully so you don't include the yeast sediment at the bottom of your jar(s). If you don't separate your finished hard kombucha from the yeast, it may develop an off-putting taste.
Once transferred, secure the bottles with regular lids and store them in the refrigerator to stop them from fermenting longer.