Find a healthy stem cutting off the plant.
Dip the end of the stem in root hormone powder (found in any garden department of a store).
Place it in a small pot with potting mix added.
You can place a little plastic tent over it to create a little greenhouse. In the spring we don't find this necessary. We placed each plant in a sunny window.Watch it and keep the water moist.
When it forms roots, move it to a bigger pot.I especially like to do propagating of variegated varieties of geraniums that I enjoyed the season before. I also have success with scented geraniums.
A good resource book I find helpful is Seeds and Propagation by Susan McClure, from the Hands-On Gardener series by Smith and Hawkin.
Lily followed me around the greenhouse Sunday afternoon and was my little helper.