If the geraniums have been growing in your garden all summer, you can plant them in pots in the same soil. Or buy potting soil and use that. Try not to remove too much of the soil that is close to the roots so that they are not disturbed more than necessary. Keep the pots by a sunny window and rotate them every few days so the plants stay even. They will grow some, but using potting soil that does not have added fertilizer will keep them from growing as much. A timer and an overhead lamp is another way to keep them overwinter. A blooming geranium in January is a great winter moral booster.   

Try using the soil that the plants are currently growing in. Garden soil may also be sold without added fertilitzer. Or use soil with fertilizer and prune the plants if they grow too much over the winter. 
Or take cuttings of your existing plants and grow new ones over winter for next year. These web pages may be helpful. 
https://sam.extension.colostate.edu/wp-content/uploads/sites/44/2018/09/Sept-8-2018-Overwintering-geraniums.pdf
https://hort.extension.wisc.edu/2024/09/26/overwintering-tender-geraniums/
https://www.extension.iastate.edu/linn/ground-how-overwinter-geraniums
https://extension.unl.edu/statewide/platte/Horticulture/Overwintering%20Geraniums.pdf