Add 2–3 slices of old sourdough bread (crusty is best) into a clean jar.
Pour over 1 liter of filtered water.
Add 2–3 tbsp of sugar (raw or brown sugar works great).
Cover with cloth and let ferment at room temp for 4-7 days. Mine took a long time due to cooler temperature here, but just be mindful and check.
Strain into flip-top bottles and let it build up the fizz for an additional day or two.
Enjoy chilled!
