Brewing lager at home
Lager is the same recipe idea as ale with one big change: it is fermented and stored cold, and it takes longer. The reward is a clean, crisp beer.
Where the name comes from
Lager comes from the German lagern, to store. Brewers kept beer in cold caves and cellars while it matured, and the yeast that survived those conditions became lager yeast (Saccharomyces pastorianus).
Fermentation
Pitch plenty of yeast into wort chilled to about 9 to 12 C and ferment at that temperature for about two weeks. Cold yeast works slowly and stresses easily, so pitch roughly twice as much as for an ale of the same strength.
The diacetyl rest
When the beer is nearly at its final gravity, let it rise to about 16 to 18 C for two days. The yeast cleans up diacetyl, a buttery off-flavour, during this rest.
Lagering
Then drop the temperature gradually to around 0 to 4 C and hold it for several weeks. The beer clears and the flavour rounds off. You need a fridge or a chest freezer with a temperature controller to do this well. The German Pils recipe gives a full schedule.