If you have a patch of ground 'out back' that has reasonable soil and good drainage, then all you need to do is dig out a patch for the Dahlias and plant them like a row of potatoes.

If you know you have any particular insect pests then take precautions but they should 'over winter' in the ground with no problems. (I'm zone 9a. I get heavy winter rain and frosts to -5C. Bedding Dahlias are fine in the ground for me and return the following season with no problems. I don't grow for the show bench but 'ordinary' Dahlias will come through easily - particularly if they are fed with good compost.)

If you can't do this then you might want to use gro-bags with some potting mix and divide the clump/s for replanting as you need.