Pitch cold, let it ferment around 50 °F (10 °C) and once the fermentation slowed down significantly and the gravity of the beer is close to its final gravity, raise the beers temperature to 65 - 68 °F (17-19 °C) for a diacetyl rest. This diacetyl rest has the effect of giving the slowing yeast a boost to finish the last sugars and reduce the diacetyl.