First of all, trim your plants excessive mass so that it cuts short to half of its size. Now dig it up using a trowel and plant it in a 6-8 inch wide pot, then move them to their new den i.e. room for winters where they get enough sun light at maintained temperature of 55 to 65 degree F. If the room does not have enough sun light, you can also use the florescent bulb. Water the pots once the soil is completely dried.
If your geraniums are already planted in pots than just trim them to half and shift inside the room as explained.
Another very popular way of saving geraniums during winter is to save their root by taking the plant into dormant form. Dig up the plant from the soil, shed of the soil around the roots. Then either put it in a paper bag and place it in a cool shady room of temperature 45-55 degrees F or just simply hung it there upside down.
There plant will shed of all its leaves and will move to a dormant form. After 2 months check the plant and put the bare root in room temperature water.
If you don’t want to keep the whole plant, what you can do is keep the cuttings for the coming year.
For this cut the 3-4 inch tender green stems and remove the leaves from bottom 2 inches. Then dip the bottom side of the stem in rooting hormone and then place the stem in a pot filed with sand which act as planting medium. Water the stem and then put the pot in a transparent plastic bag which will give the stem a green house environment for growth and it will keep it moist. Avoid the stem from direct sun light. Keep on watering the stem from time to time when sand becomes completely dry to keep it moist.
Roots will grow in 7-8 weeks and when they grow, plant them in the soil pot and place them inside a room as explained earlier.