Take about 2 pounds of apples and let them to stand until they are past their best due to the action of airborne yeast. Rinse and chop up or mash the fruit and put the pulp in to a fermentation vessel with water, bulking up to about a gallon , and seal the vessel with a water filled air lock. The yeast on the fruit will ferment the sugar in the vessel and convert it in to alcohol and carbon dioxide gas. The gas will escape through the air lock. When fermentation is complete the gassing up will cease. Strain and if necessary filter the brew. Allow the brew to stand in a clean fermentation vessel fitted with a water filled air lock until dead yeast cells and other particles precipitate down and form a sediment. When the brew is clear, siphon it off in to another vessel for subsequent bottling, with the siphon tube positioned above the sediment so that the sediment is left behind in the fermentation vessel.
For more information, check out the Wikipedia article about cider at http://en.wikipedia.org/wiki/Cider