Homemade red wine vinegar has a much more complex, subtle flavor than most of the supermarket versions. It is fabulous in salad dressings, of course, but you can also use it to make herbal vinegars, agro-dulce (sour and sweet) sauces, or to perk up lentil and bean dishes.
Start with a red wine that you like to drink. It doesn't have to be expensive, but keep in mind that if you don't like the taste of the wine, you won't enjoy the taste of the vinegar either.
Once you get a batch going, you can maintain it with just the occasional splash of wine leftover at the bottom of a bottle or in glasses at the end of a party. But for your first batch of homemade vinegar, begin with 1 bottle/liter red wine.
You will also need 1 cup of raw vinegar with the "mother." The mother of the vinegar is Mycoderma aceti, the beneficial bacteria that transform alcohol into vinegar. You can buy vinegar mothers, but probably a simpler, cheaper approach is to buy raw, unpasteurized vinegar.
How to Make It
Combine the bottle of red wine with the cup of raw vinegar in a large glass, stainless steel, or ceramic container. The liquid should only fill the container 3/4 or less of the way full.
The vinegar bacteria need oxygen to do their work, which is why you want the air space. A wide-mouthed vessel such as a crock exposes your vinegar-in-progress to more air than a narrow-necked bottle and speeds up the process.
Cover the top of the container with cheesecloth or a clean dishtowel to keep out vinegar flies but allow air in. Place the container somewhere away from direct light.
Over the next couple of weeks, a gelatinous disk will form on the surface of the vinegar. This is the visible form of the vinegar mother. This blob will eventually sink to the bottom of the vinegar and a new one will form on the surface. This looks creepy but it is actually a sign that all is going well.
When is your vinegar ready? For immediate eating in salad dressings, etc., that's up to you. Sniff your vinegar every once in a while. When it starts to have a slightly sharp, vinegar-y smell, taste it. When it is as sour as you like your vinegar to be, go ahead and strain it, bottle it, and use it.
If, however, you want to use your homemade vinegar to safely pickle food, you will need to test it to verify that it is acidic enough to do the job.