A good tie down system for canoes is a solid rack system. That might be a stock rack on a SUV or mini van, or a factory rack system designed to mount on the newer cars.
Racks today are expensive, but the security and convenience are sometimes worth the cost.
Loading a canoe by yourself is much easier with racks, it is difficult with foam carriers. But if foam blocks are your only option, here are some steps to follow to make a safe, secure tie down. 1 and preferably 2 ropes or straps around the center. Only 1 tie point is needed in the rear. In the corners of the engine compartment by the headlights, look for holes in the frame or anywhere you can tie a loop of rope that will hang out the front when you close the hood.