Bring indoors any potted geraniums and leave them in their existing pot. If you have no geraniums planted in the ground, then skip ahead to step 4.
Dig up outdoor planted geraniums from their location, and loosen as much of the soil from around the roots as you can. Use a planter that is large enough to fit your geranium's root ball, and fill the bottom few inches with potting soil.
Set the root ball of the geranium into the pot, and add in potting soil as needed around the plant. Gently work the potting soil in between open areas of roots. Fill the pot to within a ½ inch of the rim.
Place your potted geranium in a sunny window in a room that typically stays cool over the winter. A daytime temperature of 65 with nights as cold as 55 degrees is best. If the best room you have is cool but not sunny, then set up a grow light over the plant, and leave it on for 12 hours a day or more.
Use pruning shears to gently cut back the growth of your geranium by one-third. Water the geranium enough to moisten the soil. Feel the soil before watering again to make sure it is dry an inch deep into the pot, and don't water if the soil still feels damp.
Take your geraniums back outside and leave in their pots or transplant them back into the ground in the spring. Clip back any spindly looking stems to a point just after a set of leaves to encourage new growth and allow the plant to grow more compactly.