Dahlias dont actually have bulbs but they do have tubers, which perform pretty much the same function by storing the plants energy for the following year.
Now that the plants have just about finnished, carefully fork them out, as you would potato's, shake off any exsess soil and lay them out to dry a little, for about a week, idealy in a shed on some news paper, after the week is up, put them in pots or trays on a bed of peat substitute, then cover the tubers but not the crowns, keep them like this out of frost until they are ready to plant out in may after any frosts have gone. plant them about 3" deep (7.5cm) Good Luck Tbird+