Hi Paulinep the ones you really want to keep just cut them off at ground level, dig up the tubers remove all of the earth and store them upside down. What I do with the ones in the garden is when there completely over, I just cut them off at ground level and leave them, and about 75% will grow again next year. Roy.

It would be a good idea to give them a deep mulch to help keep the frost off, especially further north.
I haven't had much luck with storing them but have managed to keep some that were in the ground.

For the past two years I just picked up the pots and put them in the shed, always meaning to store them as advised. Lazy me never got round to it and the dahlias didn't seem to mind. I'll probably do the same this year.