I have been using store bought apple cider vinegar, but I have a few gallons I have been brewing since last year, I suppose it is about time to pull it out and use it (extra stuff from testing my cider press). It is almost as simple as making good hard cider, but you make sure that the cider is exposed to air which usually results vinegar rather than alcohol. You can also buy the vinegar inoculant from some places, but the natural ones work just fine too. I wasn't planning on making it last year, but a few gallons were put in a five gallon bucket and sealed, a week later it was starting to smell like vinegar so we just went with it for that batch (an experiment).