To start a Christmas cactus, you'll need to take a cutting from a healthy plant. Cut off a section that has at least two leaves and a stem. Then, remove any lower leaves and place the cutting in a pot with moist, well-draining potting soil. Water it regularly, but make sure not to overwater it. Place the pot in a warm, bright location and wait for roots to form. Once they do, you can transplant the plant into a larger pot or outdoors if you prefer.