When do I dig up my Dahlias and where to store over winter?

https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/storing-dahlia-tubers.htm

Before 1. Frost in the garage in a sandbox

You can dig them up once they have totally died back. Cut the greens off, store in a plain brown paper bag someplace cool but not cold, like a basement or pantry. Out of the sun.

I always sore mine in sawdust, get it free at big box stores or lumber yard they give it away and glad to do so. And a large cardboard drum. After they are done blooming you can cut them 2 inches from the ground, wait a week and pull the plants lay them out 3 to 4 days to start drying, then layer with sawdust in the drum. The sawdust pulls moisture from the bulb/roots.

I agree with Fiddledd! (Love the name)