Remember that Yeast Starter from a while back? Now it's finally time to introduce the yeast to the nice young beer. Up to this point we've extracted and converted starches and sugars to get them ready for the main event. The couple hundred billion eager yeast cells are nice and hungry as well as ready and willing to create the alcohol that makes this process worth the time and effort.
If you're not planning on using a yeast starter, don't worry. A WYeast smack pack that has been working for a day or two should still do the job on lower OG beers (around 1.050 and below), however, the fermentation cycle may not take off as fast or may not be as vigorous as it would be with a starter.
Whichever method is chosen, simply pour the yeast slurry into the fermenter and place an airlock or blow off tube on top. Now the magic can start happening.