Pruning a Christmas cactus is not difficult, but it's important to be careful when cutting back the plant. First, identify the old growth that you want to remove. Then, use a sharp pair of pruning shears to cut it off at the base of the stem. Be sure to leave the younger, healthier growth intact. After you've removed the old growth, fertilize the plant with a balanced fertilizer and give it plenty of water. That should help it grow strong and healthy.