Christmas cactus cuttings can be started by taking a stem cutting from an existing plant and placing it in a pot filled with well-draining soil. It is best to take the cutting in the spring or fall when the plant is actively growing. Make sure to choose a stem that is at least six inches long and has two to three sets of leaves. Gently remove the cutting from the plant and cut off any leaves that will be below the soil line. Let the cutting callous over for a few days before planting it in the pot. Water the pot regularly and keep it in a bright location.