The usual way is to wait until foliage is hit by first frost and then cut down stems leaving about 8inches. Loosen them from soil and shake off soil carefully. Place upside down so the stems drain for a few days and then store in dry to slightly damp compost in cool dark place. Having said all that I have already lifted some dahlias as I need space for bulbs. ( I have bulbs in open weave aquatic baskets which I bury in the garden but can dig up after flowering to keep the foliage growing until it dies back naturally. This frees up the space agin for planting dahlias again.)
Last year I experimented with just damp newspaper above and below the dahlias rather than compost and had no problems getting them to grow this year.

Thank you so much and I will follow your guidance to the letter!
It was kind of you to help me out with this.

My method is similar, except you don't need to wait for them to be blackened by frost.
The best way, if no frost has occurred before the 1st November, is to cut them down as above but not 8 inches, leaving just 2-3 inches of stem, above the tuber. Leave them for 2 weeks before digging them up. This allows "eyes" to develop for next years growth. After digging, clean off all the soil with a hose spray, cut away all the fine roots, then get a long screwdriver or similar and screw down the stem and out through the bottom of the tuber. This allows water to drain out from neck of the stem.
Should you wish, you can dust the tubers with flowers of sulphur, before storing in boxes of dry peat/compost. Must be dry. You must store them somewhere where room temperature does not go above 50'f, and stays above freezing. Ideally, as cold as possible.