Place berries and sugar in a half gallon jar.
Fill jar with water, leaving 1” headspace so you have room for stirring.
Stir to dissolve the sugar. It might not fully dissolve yet, but it will by day two or three. Cover the jar with a clean cloth secured with an elastic band. Place on the counter for 4-5 days. I like to place it on a baking sheet or tray, just in case it bubbles over (it usually does not).
Give it a stir daily.
By day 4-5 give it a taste. If it’s still quite sweet and fizz has not yet formed, let it ferment for another day or two.
Once it is nice and bubbly and sweetness has diminished a bit, strain off solids and transfer to clean bottles for the second stage of fermentation. Air tight swing top bottles work great. Note: I’ve never needed to go past 6 days. Room temperature plays a big part. On warmer days it will ferment faster than cool days.
Let the bottles sit at room temperature for another 2-3 days, until bubbles are to your liking. Be mindful of carbonation building when you open to check. I open and close VERY quickly, so gases can release a little at a time. It also does not hurt to do this outside just in case. Remember to open and close very quickly, a few times. It helps to control the effervescence.
Once you are happy with where soda is at, place bottles in the refrigerator. You really do get a feel for “doneness” with a few reps under your belt. It will keep here for several months in the fridge. Enjoy the fizz!