Remove the tubers from the dry compost and put them in pots with the growing tip just below compost level. Water them well and leave them in a warm place like a greenhouse or against a sunny house wall.
They should soon start into growth, but don't keep them wet otherwise the tuber can rot before new growth starts. Then plant them out when the plants are a reasonable size and when there is no more risk of frost.
Don't forget if you put the tuber in compost in shallow tray over gentle warmth, your old tuber will produce lots of shoots which you can cut off as low down as possible and then put into a well drained gritty compost. Put a polybag over them and they will root in a couple of weeks to give you lots of new dahlia plants.
The easiest thing of all to do is simply to plant the dry tubers in the garden about four inches deep and they will start into growth when the weather warms up. That way your flowers will be much later of course.