Do not soak. Plant them up to the crown....where the stem meets the tuber.... in a big enough pot....usually about 5"...using moist compost. Then keep it in a warmish place.
Don't overwater it, the tuber can rot if too wet.
Shoots should appear in about 2-3 weeks. You can then water as and when.
Planting in your final tubs can be done straight from these pots end of May, when in theory no more frosts will be forecast.

Lazyhoe, I do hope you don't mind if I partly hijack your question because mine is very similar. A neighbour has given me several dahlia tubers, and I've never grown them before. Only one of them had any sign of a shoot, but I've already done what has been suggested above. I suspect that these are tubers that never got planted LAST season, but they still seem quite firm. I, too thought soaking might have been a good idea but decided not to. What chances does anyone think I have of getting plants from these?

If they are firm then there is a very good chance they will grow. Dahlias need soil/air temperatures above 50'f to start into growth. Hence, when storing, keep below these temperatures.
Tubers from your own garden take longer than those bought from shops. They are tubers grown in 5" pots to restrict the size of the tuber. Garden tubers are best started off in trays 3" deep.
I have written a blog giving details of such trays and my methods for propagating.