Put the raisins and water in a sterilized air-tight jar.

Place the jar in a sunny spot near a window until it ferments. As long as it's warm, anywhere is fine.

Stir the contents of the jar once a day and open the lid to refresh the air. You should be able to hear fizzing once it starts to ferment.

Although it depends on the temperature, in 2 to 4 days, it should start to fizz. If the fermentation (fizzing) takes longer than 4 days, add sugar.