In the colder climate zones, dahlias have to be dug up in the fall and stored for the winter. I have been digging and storing tubers for many years. I have tried many different methods. Here is what works best for me.
1. After the first hard frost (usually mid to late October in Michigan, zone 5), I cut off the tops of the dahlia plants and wait for 1-2 weeks. This allows the tubers to start forming new buds and makes it easier to divide them. A tuber with no bud will not grow.
2. Dig up the tubers and rinse them off. If you damage the neck of the any of the tubers, they probably cannot be saved. If you happen to cut a tuber in half, it will normally store just fine.
3. Wash and cut (I divide in the fall to minimize storage space). If the new buds are not clearly visible, you will have to make a best guess. The goal is to have at least one bud per individual tuber. The first photo below clearly shows the bud.
4. Allow the tubers to dry for 4-6 hours or maybe even overnight. I usually do this inside (think shade).
5. Mark each tuber with a permanent marker. If you have trouble doing this, the tubers are probably not dry enough. I abbreviate the Dahlia name.
6. Wrap each tuber in plastic wrap (Saran Wrap). One tuber per sheet makes it easier to deal with them in the spring. However, you can wrap 3 or 4 together, making sure each tuber is separated with plastic wrap.
7. Pack the wrapped tubers in bags or boxes and store in a cool dark place (basement cellar).
8. Settle in for winter. Sometimes I check the tubers, but most of the time I just wait until spring planting. If you have any tubers that have spoiled, you can deal with them at that time.
I usually do 12-16 tubers per day. I can dig, wash, and cut this number and be done by noon. The tubers are dry by around 6:00 PM, so I complete steps 5-7. I repeat steps 1-7 for several days until I reach step 8. This is the most rewarding step!
Also worth mentioning, I have a good pair of waterproof gloves and a great garden HELPER!