This spring I sold a lot of cuttings I had taken from my tubers. I gave a set of instructions to each person that covered, first, what to do with the cuttings and then how to care and eventually store them. It pretty much covers a year in the life of a Dahlia enthusiast.

I will be revising it based on this year's experience, but not until I've lifted my pot tubers (which should be over the next month.)

So have a look at this link; Instructions: How to care for Dahlias from cuttings and then feel free to ask more specific questions.

To answer your question specifically:

Let the weather kill your Dahlia above ground. Once it has been frosted a few times, the plants will eventually turn black and collapse. Your tubers will be fine at this point. Cut the stalks 6" above the ground and remove the dead plants.

Leave the tubers in the ground for another week or two. Only lift them if there is a chance of many inches of snow, or a severe frost warning that would freeze the ground. Typically, you shouldn't have a problem leaving them the extra week unless you live in an extreme climate.

After the one or two weeks, lift the tubers. They will have formed "eyes," or the very beginnings of new sprouts. These form after the main stalk is cut. They are your guide to where to cut your tuber clumps to separate individual tubers. They're essential for cutting.

There's lots of ways to store them over winter. I use the "saran method" for individual tubers. See this past thread; Saving tubers? I thought there was a FAQ... for a discussion about various ways to store.

Cheers,
Russ