The easiest way I've found is to add about 1/4 cup of organic apple cider vinegar with mother to a gallon of fermented cider. Cover the vessel with cheese cloth and put in a dark place for about 2 months. (the taste will tell you when it's done) Room temperature works fine for this. A SCOBY will form and grow under the surface. Transfer the vinegar from under the SCOBY to bottle. The SCOBY can be saved in a jar, covered with some of the vinegar and stored in the fridge for future use.