To fertilize a Christmas cactus, you should use a liquid fertilizer specifically designed for cacti and succulents. Apply the fertilizer once a month during the growing season, typically in the spring and summer. Dilute the fertilizer in water according to the instructions on the package, and use a watering can to distribute it evenly over the soil. Be careful not to over-fertilize the plant, as this can cause leaf drop or root burn. Make sure to allow the soil to dry out between waterings, and do not fertilize during the winter months when the plant is dormant.