I don't do it the text book way but it works for me and I rarely lose any.

When the frosts hit them after they have finished flowering I dig them up , trim off the shoots and leaves and let them dry out indoors. Then when dry clean off as much of the dry earth as possible and wrap each one in newspaper and store in a cardboard box in the garage. And label each one because you will forget which is which.

Last winter with my biggest dahlias I left them in the ground and covered each one with a large mound of compost. They have come up and are doing well right now.

many thanks Inverglen.

After they are blackened by frost, cut the tops off to about 2 inches above the crown, dig up the dahlia, wash all soil off with a hosepipe spray gun, then get a screwdriver and skewer down through the stem and through the bottom of the tuber, then dry off in a frost free place for a few days.
When dry, store the tubers in trays/boxes of dry peat/compost, buried up to the crowns only.
They must be dry before storing. As long as the place they are stored is above freezing and below 50'f, they will survive the winter. Mine are stored in the greenhouse under the staging, but I have a heater that is set to frost guard only so temperatures go no lower than 40'f. Cool garage is fine.

Many thanks