We cut the bread. We make crackers. We fill them with a three-liter jar of boiled hot water.
Add sugar. Add yeast. Stir the contents of the jar.
We close it with gauze folded in several layers, fix it with an elastic band. We put in a warm place and cover with a cloth.
We stand kvass for 2 days. Then put it in the refrigerator. When the kvass is cooled, strain it and we can add sugar to taste.