Make a light simple syrup (2 parts water, 1 part cane sugar). To do so, heat sugar and water in a small pot on the stove until sugar dissolves. Add fruit, herbs, spices, and flavorings you want to include (see flavor ideas below). Cover the pot and let it simmer until the flavor is infused. Add more water if too much evaporates. Let cool and strain into a clean container. If you have berry or flower syrups you made previously, you may use these instead.Add the yeast to a cup of warm water then add a pinch of sugar to activate it. You can use regular baking yeast, or if you are buying yeast for this purpose, champagne yeast may yield a cleaner flavor. Let this mixture sit for 10 minutes then stir it to make sure it is well mixed.Set out 2 1-liter recycled plastic soda bottles. (Recycled plastic soda bottles are great for the first fermentation because they can handle pressure, don’t shatter if they become overpressurized, and have screw caps that can slowly release gas.) Put ½ of the yeast water in one bottle, the other ½ in the second bottle. Do the same with the simple syrup. Use the remaining spring water to top off the bottles but leave 1 inch of headspace at the top so the mixture can expand. Taste the soda for flavor and adjust as needed.Let the soda ferment at room temperature for 12–48 hours. You can tell it is finished if the bottles’ plastic is hard (not squishy) from built up carbonation. Once it is ready, either transfer the soda into glass bottles with tight-fitting lids or keep it in the plastic bottles and release the carbonation. Refrigerate.