Cindy Haynes, Department of Iowa Horticulture wrote the following.

"To overwinter geraniums in dormant storage, dig up the entire plant before frost and gently shake the soil from the roots. Place the plants inside open paper bags or hang them upside-down from the rafters in a cool, dark location for the winter. Ideally the temperature should be between 45-50 F. Two or three times during the winter, take the plants out the bags or down from the rafters and soak the roots in water for 1 or 2 hours. At this time, inspect the stems. While many of the leaves will die and fall off, the stems should remain firm and solid. Discard any geraniums with shriveled stems, since those plants will most likely die. Pot up healthy dormant geraniums in containers in late March or early April. Water plants thoroughly and cut back the dead stem tips. Place potted plants in a sunny window to initiate new growth. It often takes several weeks for plants to initiate growth after dormant storage."

https://hortnews.extension.iastate.edu/2004/9-17-2004/geraniums.html