As the old saying goes, "Men don't make beer. Men make wort. Yeast make beer." Wort (pronounced vurt) is the boiled and hopped pre-beer liquid that requires the most human effort (you can read more about this in the 10-Step Brewing Guide).
This is as far as an individual can go in actively "making" a beer. The next step is allowing fermentation to take control. Though less "hands on" than the brewing process, the fermentation stage is equally important in determining the final flavor of the finished product. Following the guidelines below will ensure that you help your yeast "make" the best homebrew possible.
Temperature control is key.
Even if you don’t have a temperature control system (such as a dedicated fermentation refrigerator or a precisely temperature-controlled room) it is still worthwhile to keep your fermenting beer as close to a target temperature as possible (as prescribed by your recipe). Some sources state that yeast get stressed and produce off-flavors if they experience temperature changes of more than 5° F in one day.
Most likely a room’s temperature will change by this much from daytime to nighttime. Thus, it’s best to place the fermenting beer in a cool, insulated location, such as in a closet or in the back corner of a basement, away from direct sunlight.
Since most environments are warmer than desired, placing the fermentation vessel in shallow water with a wet towel or t-shirt covering it can help lower the fermenting temperature a few degrees, and help buffer against temperature shocks.
Keep it clean.
During the fermentation stage, you may want to open the fermentation container to retrieve a sample for a density reading, or perhaps just for a taste. In doing so be sure to sanitize your equipment (a sampling container or spoon) and to only allow it to contact the fermenting beer when necessary.
It's best to retrieve a larger sample than necessary just once than to open the fermentation container multiple times. Another instance when infection may occur is during transferring - some recipes work best with a transfer of the beer off of its yeast sediment into a new fermentation vessel, after primary fermentation of around 3-5 days.
Again, be sure at all materials coming into contact with the beer are sanitized, preferably with a no-rinse sanitizer such as Star San.
Avoid air contact as much as possible.
Once your wort has become beer, oxygen becomes the enemy. Luckily oxygen bound to carbon in the form of carbon dioxide does not negatively oxidize the beer, and even pleasantly carbonates it. When accessing the beer for retrieving samples, transferring the beer to a new vessel, or bottling the beer, avoid excessive splashing or sloshing of the liquid, since this will push ambient oxygen into the solution.
Just be deliberate and quick with your beer contact and be sure to seal everything when finished.
After approximately three weeks, your beer will reach the optimal state for consumption. If you're having a get together, feel free to open the fermentation bucket and fill glasses straight from the vessel (a la English cask beer).
After opening the fermenter, if you're not able to finish all your brew within a day or so, bottling becomes the next best option to preserve your beer.