Cover any kind of chopped fruit and/or fruit peelings & cores with water, in a clean wide mouth glass container. This can be scraps leftover from home canning, pie making, or general mayhem involving fruit.
Different fruits yield their own special gourmet flavors. A pear canning spree usually results in a nice salad vinegar with a delicate undertone. Apple peelings make a more robust vinegar. Experiment and have fun.
If possible, use rainwater, distilled water or water from a well or spring. Chemicals in tap water may interfere with the natural bacterial action that is needed to make vinegar. Cover the container securely with a cloth to keep out insects while allowing contact with air. Keep it in a warm dark place for a few months, stirring now and then to allow the topmost layer to work with the rest of the mess.
The wild strains of vinegar-making bacteria present in the air should colonize and feed on the sugars and starches in the liquid. After a few weeks, you will notice a vinegary smell. Allow the liquid to ferment until desired strength is achieved (smell, taste). Strain and pour into clean bottles.
For faster and better action, add a cup or so of natural unfiltered vinegar (from health food store) to the water before fermentation takes place. This promotes rapid growth of the good bacteria, while discouraging unwanted bacteria that could spoil the batch.
You can make vinegar from just about anything that contains starch or sugar: Fruits, fresh or frozen fruit juices, berries, grains, roots, or even a 10% sugar water solution.
Do not use canned or bottled fruit juices, as they contain chemicals that prevent fermentation.
Read more about the use of vinegar here