The diagram below shows a simple schematic diagram of a do-it-yourself wine fermentation setup. Most wine beginners use the use the traditional cotton plug method. Its a big NO.
Fermentation occurs in absence of oxygen. Oxygen entry may allow the growth of foreign acetic acid bacteria. The setup below is ideal it allows escape of carbon dioxide produced by the action of yeast but prevents the entrance of oxygen.
You just need an extra rubber plug, piece or rubber tubing and small bottle of water.