The procedure is simple but to minimise the risk of spoilage you should work quickly in scrupulously clean conditions, using the minimum of equipment. You will need a carton of grape juice with no preservatives, a soup bowl, a kitchen knife, a teaspoon and and a small plastic bottle of drinking water at room temperature, 200 or 250 ml is ideal.
- Put about ten good ripe grapes in the soup bowl. Lightly rinse them with the drinking water. Pour away the water (or drink it if you like) leaving the grapes in the bowl.
- Chop the grapes fairly small then lightly crush them with the teaspoon. They should still look like chopped crushed grapes, not grape purée.
- Spoon the crushed grapes into the empty water bottle. Add enough supermarket grape juice to more than cover the pulp. The bottle should be no more than half full.
- Replace the bottle cap and tighten it. Shake well, then loosen the cap just enough to let gases escape. Leave the bottle in a warmish place for fermentation to start.
Finally, having made a successful natural yeast culture once, you never need to do it again. Instead, you can simply salvage a little of the sediment from your finished wine and use that to start your next batch.