When you're about 75% of the way to FG (usually about 1.020 or so), you can raise the temperature up by 10 degrees or even more for the diacetyl rest. You can do it in the fermentation chamber, or out of it, it really doesn't matter at all. Let it sit at least 48 hours once it reaches 60 degrees (or whatever your rest temperature will be) and then check the SG to make sure the beer is completely finished. You can also taste for diacetyl at that time to make sure you don't have any. In very small amounts, diacetyl will present as just a slickness on the tongue or a slight oiliness. If you have ANY hint of that, keep the beer at the diacetyl rest temperature until it's gone.
After that, the beer can be racked and the lagering can begin.
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