Add cold bottled drinking water (do not use tap water! there are chemicals there that might kill fermentation process) to almost the top of the pot.
Dissolve 60ml rye sourdough starter in 1 cup of kvass extract, stir, and pour it into the pot.
Keep at room temperature (72-74F) for about 2 days. It takes 12 hours when the temperature is 86F. At lower temperatures, fermentation process takes longer.
...in 12 hours, 72F, the bread will float to the top and the surface of kvass will be partially covered with bubbles.
...in 24 hours, 72F. Your kvass is ready to bottle when its taste is pleasantly sour, with an appetizing aroma.
Prepare two pint-sized clean screw-cap or flip-top bottles and funnel.
When bottling kvass, make sure the sediment on the bottom of the jar where kvass was fermenting is not disturbed. To carbonate kvass during the next step, conditioning, feed it with agave syrup right before bottling. Add 2-3 small raisins per 1/2 quart of kvass. Seal the bottles.