To prune a Christmas cactus, start by identifying the plant's main stem and then cut off any lower stems or weak growth. This will help the plant focus its energy on its main stem and promote healthy growth. Next, remove any damaged or dying leaves, as well as any stems that are growing inwards towards the center of the plant. Finally, trim back any long stems to keep the plant's overall shape in check. Make sure to always use sharp, clean pruners to avoid damaging the plant.