Fruit skins hold wild yeasts - use wild or organic.
1 litre
Peel and core 2 to 3 apples, pears, quince etc.
Eat the fruit and use the peel and cores for the kvass or include some of the fruit.
1 litre of water, chlorine-free, boiled and cooled or filtered.
2 tablespoons sugar or honey. Sugar ferments more reliably.
A glass jar of about 1.5 litres so there's headspace for fizz.
A breathable cover. Improvise using cloth, coffee filter, paper towel or a loose fruit-fly-proof cap.
Optional: Rye flour (malted is better but not essential) and a half teaspoon of sourdough starter.
Mix up the ingredients and stir to dissolve the sugar. Leave about 3 cm of headspace.
Cover and ferment at 20 to 25 °C away from direct sun.
Once or twice a day, stir with a clean spoon to keep the fruit or peelings submerged and to help the yeast spread.
After 4 hours, there should be small bubbles.
By day 2 to 3, it should smell pleasantly fruity and slightly yeasty.
Taste daily: it should be lightly sweet, tangy, and a bit fizzy.
When you the flavour is good, often on day 4, strain out the fruit scraps.
Pour the liquid into bottles with tight lids and allow to preasureise to give a bit of fizz.
At all stages ensure that gasses can escape to prevent "bottle bombs".