How To Carbonate Your Own Soda

1. In a clean glass bottle of any size with a tight-fitting lid, combine syrup of choice and tap water in a ratio of 1:2— for example, a 750 ml bottle can hold a little more than 24 ounces, which means 8 ounces of syrup and 16 ounces of water.
2. Add to the mix a scant pinch of active dry yeast, cap tightly, and shake well to combine. Label and date the bottle. Allow it to get gassy at room temperature for 1 to 3 days until carbonated. 
3. Once the soda has achieved the right level of bubbles, move the bottle to the refrigerator and let the soda chill.

Note: The best way to test for carbonation is to create one small “testing” bottle of soda that should be checked daily for proper carbonation. To do so, decant 1 to 2 ounces of your soda into a separate bottle, rescrewing the cap tightly after each check.