The best way to protect your Christmas tree from a cat is to keep it out of their reach. If you have a small cat, you can try placing the tree in a corner where the cat cannot easily reach the ornaments or climb the tree. You can also try placing a barrier around the tree, such as a pet gate or a tall piece of furniture, to block the cat's access. Additionally, you can try spraying the tree with a cat repellent spray or hanging aluminum foil from the branches to deter the cat from climbing the tree.