Firstly,
and most importantly, if you want to keep the top of the tree, turn it round
so the base of the trunk faces forward. If you do it the other way round
the wind will whip the thin top of the tree to shreds.
Wear Protective clothing. A thick jacket and gloves are really necessary as you will need to 'get in there' to tie the tree down tightly. Consider protecting the roof of your car with cardboard, carpet or blankets before hauling the tree up there. Use good rope or elasticated bungee cords tightly wrapping them across the top of the tree and under, securing them to each other and the roof rack.
Each bungee should be locked down at one side, looped ofer the tree's central trunk and secured at the other side of the car.
The big mistake with a long rope is to tie one end and then loop it back and forward across the car and secure the other end. This leaves you with only two secure points and you may lose your tree and endanger other drivers. Better to lock down the ropeat as many points as possible.