The best way to cut back a Christmas cactus is to trim it back by about a third, starting from the top. This will encourage new growth and help keep the plant looking healthy and vibrant. Be sure to use sharp, clean shears and make clean cuts at a 45-degree angle. After trimming, water the plant generously and fertilize with a slow-release fertilizer. With proper care and pruning, your Christmas cactus can remain beautiful and healthy for years to come.