To tie a Christmas tree to a car, you will need to secure the tree firmly to the roof of the car using straps or ropes. Make sure the tree is tied down tightly and securely so it doesn't move or fall off while you're driving. Additionally, make sure to tie the tree down in a way that doesn't damage either the tree or the car. Once you have the tree tied down, make sure to keep an eye on it while driving, especially when making turns or going over bumps in the road.