3 cups not-so-hoppy beer
1 jar mother of vinegar, which is a combination of a SCOBY (symbiotic colony of bacteria and yeast) and raw vinegar
Combine all ingredients in a large jar, crock or oak barrel with a wide opening on top. Cover the opening with muslin cloth and secure cloth with a rubber band. Store in a warm, dark place for 30-60 days.
After 30 days, smell and taste your vinegar. It should smell and taste like vinegar, and you may see a new "mother" forming on top. If it smells like nail polish remover, not to worry, it simply needs more oxygen to complete the fermentation. Swirl to aerate and let it continue to ferment until it tastes right. If using an oak barrel, let it ferment for a minimum of 3-4 months to enjoy the flavor benefits from the oak.
Bottle your vinegar in dark bottles or a ceramic vessel with very little head space and seal with a cork or lid. Vinegar, like fine wine, gets better with time.
Before bottling, reserve some of your vinegar to use as the mother for the next batch. The basic recipe for vinegar is 2 parts beer and 1 part mother vinegar. Your reserved mother vinegar is one "part." Adjust the amount of water and beer accordingly.