I didn't know whether to put this here or in Food Processing. It's both I suppose, but you have to brew cider first to make this! Some might consider this a waste of cider, but in our house good vinegar is indispensable in the kitchen so it was well worth setting a DJ of cider aside to make some.
Acetobacter is a bacterial strain which turns alcohol into acetic acid. You really don't want it in the home brewing or winemaking environment, which is why we ferment under airlock, store in airtight bottles and keep the dreaded fruit fly (which carries
acetobacter) away if possible. To make vinegar, we expose an alcoholic solution to conditions which encourage acetobacter growth and let it do its thing.
There is more info about it on the blog (link below), but essentially we brewed our own cider from apples, then set a couple of half-full demi-johns (to maximise surface area exposed to oxygen) added a spoonful of unpasteurised ACV (for the chooks!) to each and left in a warm place for a few months.
Hope you find this useful and try it!
More info here!