Add the Kefir Soda Starter packet OR kefir whey to the juice in a glass bowl and stir until dissolved. Fill the rest of the bottle with water leaving an inch of room at the top.
Allow it to ferment for 3-7 days or until bubbly and fizzy. If your home is 75°F or warmer, it will usually take 3-4 days. Colder homes can take 5 days or longer, but you'll need to check it by popping the top to see if it's getting fizzy. It will start to bubble out the top if it does.
When it's done, you can pour it into a glass and enjoy it right away or place it in the fridge. It will continue to ferment in the fridge, so burp it daily and consume it within 1-2 weeks for the most benefits and the best flavor.