Red wine vinegar is made by fermenting leftover wine with a vinegar mother or a small amount of nonpasteurized vinegar. The fermentation process may take as long as three months.
A vinegar mother is a film of bacteria that sometimes forms during the fermentation process. The mother may be reserved and used in new batches of vinegar. Mothers are also usually available for purchase from beer- and wine-brewing stores.
To make a large batch of vinegar, pour 1 bottle of wine and 2 cups water into a wide-mouthed glass container with a vinegar mother. Cover with two layers of cheesecloth, seal with an elastic band, and let sit in a cool, dark place. Once a week, feed the mixture by adding 1 or 2 cups of red or white wine. Do not pour the wine directly onto the mother. After about 3 months, strain the mixture into tall, airtight containers.
To make a small amount of vinegar from leftover wine, pour roughly 1 cup wine into a wide-mouthed glass container, then add 3 tablespoons of unpasteurized vinegar. Stir vigorously, and cover the top of the container with a towel or two layers of cheesecloth sealed with an elastic band. Let it sit at room temperature, stirring at least once per day. After about 1 month, taste the mixture, and move it to an airtight container if finished.