August, September and October--depending on your local climate--are ideal months to take cuttings from your healthiest geranium plants. Young cuttings should have a minimum of two weeks to establish themselves and a rudimentary root system before being expected to overwinter. Even when weathered indoors, temperature swings can be hard on young geraniums. Taking a few minutes lay out supplies before you begin the cutting process will help you finish quickly, with the healthiest plants-to-be.
Spread out old newspaper or garbage bags to catch dropped dirt and plant debris, if working inside or if you need to protect your outside work surface.
Fill the containers for root cuttings to within 1 inch of the rims with one part peat and one part sand. If you're rooting cuttings individually, place them in 3-inch-diameter pots. If you're creating the illusion of a robust plant by grouping multiple cuttings together, use a larger pot up to 10 inches across.
Select one (or more) of your healthiest geranium plants as a source for cuttings. Robust growth and numerous brightly colored, well-shaped flowers are indicators that a plant will make a good donor. Use healthy, leggy stems for cuttings.
Slice each stem off the parent plant, 1/8 inch or less, beneath a leaf node. Trim the parent plant's remaining stem back to the next leaf node down, if you like. Pull off any flower buds and extra leaves, including the tiny leaves on either side of the nodes, from the bottom inch of the cutting as they will rot if allowed to remain on the stem beneath the soil level.
Insert each stem into the pots you prepared as deeply as necessary to stabilize them without packing down the soil around them. Remove any remaining leaves or flower buds that may end up below the soil level if you missed them in step 4. Roll the stem ends in rooting hormone before planting, but this is not necessary to produce healthy new plants from geranium cuttings.
Water as needed to keep your cuttings barely moist, making sure to point the watering can spout away from the geranium stems. Allow them to absorb water from dampened soil around them. Maintain the cuttings in a controlled environment with bright but indirect sunlight and nighttime temperatures as close to daytime temperatures as possible. Cuttings can be transplanted once they show signs of new growth--usually about two weeks.