Keg conditioning (natural carbonation) utilizes yeast's natural ability to convert sugar to CO2 in order to carbonate your beer! Although it does add a considerable amount of time to your brewing process, many brewers swear by this time honored tradition and claim it provides a different mouth feel compared to force carbonated beer.
In order to naturally carbonate your beer, you'll need to begin once primary fermentation has completed. At this point, you'll need to make food (sugar water) for the yeast. Do this by adding your measured amount of sugar to a cup of water and bring to a rolling boil for 5 minutes. The amount of sugar needed varies by recipe, so check this beer priming calculator for to compute the ratio for your recipe. Directly following, remove from heat and pitch to your empty and sanitized keg. This helps to mix the solution throughout the beer.
Once your solution is in the keg, use your sanitized siphon to rack your beer from the fermenter to the keg. Be careful to siphon only beer and do your best to avoid picking up particulates from the bottom of the fermenter.
Your keg is then ready to be sealed and placed in a temperature controlled environment for two weeks, between 65-75ºF. When the two weeks have passed, chill the keg to proper serving temperature, and put it under pressure. Once pressurized, pour out a glass of beer for inspection.
The first few pours could have particles or a heavy yeast flavor, so tossing a glass or two is a common occurance. However, some people enjoy this part and will gladly drink the first glass. Cheers!