The third essential step in winemaking is to additional ingredients to strain off the liquid from the pulp, put it (the liquid) into a secondary fermentation vessel (a carboy or jug), fit a fermentation trap (airlock) on the mouth of the bottle, and allow fermentation to proceed at 60-65 degrees Fahrenheit until all bubbling ceases (after several days to weeks).
Transfer does not mean rack. While the solids are strained off and discarded, the liquid and lees are poured through a funnel into the secondary. The lees are important at this stage because many of the live yeast cells will have settled into the lees. Without them, fermentation will get very sluggish or stick (stop altogether). The best procedure is to stir the wine to get the lees into suspension, then pour the liquid through a large funnel into the secondary. If the funnel has a fine-mesh screen insert, use it to filter out the gross lees (large bits of pulp). The fine lees, containing the yeast, will pass through the mesh.
The timing of the transfer has long been the subject of debate. In most of the 20th century, the commonly accepted procedure was to ferment for three to five days and then transfer regardless of specific gravity. The set number of days was usually arrived at by averaging the time it took to ferment enough sugar to reduce the specific gravity to between 1.050 to 1.030. The idea was to get the wine under airlock so it could begin a true anaerobic fermentation, or fermentation without access to oxygen. The reason for this is because this is when the yeast converts sugar into alcohol and carbon dioxide while extracting energy from the transformation. More recently, it was discovered that yeast create a micro-anaerobic environment around themselves as they submerge by using whatever oxygen atoms are trapped in the spaces between molecules surrounding them. Only when they float up to the surface are the possibly exposed to oxygen, and after the first 48-72 hours atmospheric oxygen has been replaced with carbon dioxide created by fermentation. Thus, the old idea of getting the wine under airlock quickly to create an anaerobic environment was shown to be mostly irrelevant and a new practice was adopted.
Today the accepted practice is to keep the wine in the primary until the vigorous fermentation subsides. This normally occurs at around specific gravity 1.010. As the vigorous fermentation subsides, the production of carbon dioxide slows considerably and oxygen is able to migrate down through it to the surface of the wine. Free sulfur dioxide still protects much of the surface wine from absorbing oxygen, but sulfur dioxide is a gas and slowly escapes the wine. The intermolecular spaces vacated by the sulfur dioxide are filled either with carbon dioxide rising through the wine or by oxygen scavenged from the atmosphere by the wine -- a natural ocurrance. So, as the vigorous fermentation subsides, the wine becomes more vulnerable to oxygen uptake. This is the best time to transfer the wine to secondary.
The best way to accomplish the transfer without exposing the wine to more oxygen than is necessary is to tilt the funnel so that the wine escaping it slides down the inside of the carboy rather than drop free-fall through the air to the bottom. This may require the tilting of the carboy itself, but simply tilting the funnel means this is a two-person operation. An alternative procedure is to fit a hose or tubing to the bottom of the funnel that extends to the bottom of the carboy. This, too, minimizes air contact.
Another way to minimize air contact while transferring the wine to secondary is to fill the carboy with carbon dioxide. A small bottle of compressed CO2 will do this most efficiently. As the carboy fills with CO2, it settles and fills the carboy from the bottom up, pushing oxygen-laden air up and out of the carboy. The trick is to not fill the carboy too quickly, thereby allowing the CO2 to settle. Argon is another inert gas that can be used in the same way to protect the wine against exposure to oxygen-laden air, but both methods are really unnecessary if care is taken in tilting the funnel appropriately or using an extension hose or tubing as described.
It sometimes happens that everything in the must is just perfect for the yeast and they ferment the must to absolute dryness in only two or three days. When this happens (a hydrometer reading will show the specific gravity at less that 1.000), go ahead and rack the wine into the secondary and slap an airlock on it immediately.
When the fermentation in the secondary stops -- that is, when positive pressure inside the carboy stops pushing bubbles through the airlock -- it is essential to use the hydrometer to ensure fermentation is finished rather than stuck. This is only common sense, but it is surprising how many people forget it and assume the fermentation has stuck rather than finished.