You cut down a Christmas tree by using a sharp axe or saw to cut through the trunk at the base of the tree. You should aim to cut it down at a 45-degree angle so that it does not fall over or cause any damage to the surrounding area. It's important to be careful when cutting down a tree, as it can be dangerous. Make sure to wear protective gear such as gloves, goggles, and a hard hat when cutting down the tree, and always make sure to have another person present to help you.