When I worked commercial construction, I learned to tie things down from roof top racks to flatbed trailers.
Those ratcheting straps are a God send  and make the job much easier for people with limited experience.

Not everyone has them and IF you don't have them or those flat nylon canoe straps with the cam lock buckles, your going to be going old school and use ropes alone.

Whatever you do, don't tie your expensive canoe(s) down with cheap clothesline rope that stretches when wet and doesn't have enough load strength to do the job. I see people using Para-Cord becasue it's rated #550 in the military spec style but that's a poor choice as well. It's strong as hell, but it stretches under load and you don't want that.

Buy some decent rope at the Home Depot and check it's load strength and just as importantly whether it will stretch significantly. Most rope sold precut has a chart on package that tells you what it's good/bad for, pay attention to it.

A great knot for bow/stern lines as well as a line(s) over the canoe, is the Trucker Hitch. It's simple to tie and re-adjust if needed and you get a mechanical advantage with it. That means when you snug it up and tie it off you don't lose tension like you do on many other styles of knots.
It's popular for a reason. Everyday truckers secure loads with it and trust it.
Something worth learning IMHO.

Excellent around camp for guy lines/tarp ropes, just about any camp application.
There are other knots better for specific tasks, but for an overall knot that is easy to tie and untie, the Trucker Hitch is hard to beat.

When traveling any distance with my boats, I have always used  Thule or Yakima racks with the canoe  brackets.  Once the brackets are fitted to the gunnels and strappend down, those boats don't budge. The canoes brackets are worth there weight in gold! I then tie them down at the bow and stern.  Solid as a rock.  They have gone over the Mackinaw Bridge and the Duluth Bridge in high winds and don't even budge.   

On the other hand, I don't paddle aluminum canoes.  If I did, I probably wouldn't care as much if it came off the car (as long as it didn't hurt anyone).  From the way I see people drag those boats over the rocks in the back country, I'm surprised people don't just drag them home behind the car.   

great idea, and you're right, (You need to Login or Register to view media files and links) are simple enough to make

People, canoes, and cars are so various that it's hard to describe a system that will work for everyone. On the truck I use to carry my canoe I finally broke down and drilled holes in the ends of the bumper and mounted stainless steel eye bolts. That gives me some great tie down points to secure the canoe at the bow and stern. Along with a loop that goes around the belly of the canoe and is secured at each end to the forward shackle of the leaf springs. What ever system you come up with I would suggest that you try it out at home, put the canoe up there and drive around a bit. Let the wind and motion of the car play with your lashings and then pull over and check things over. It's always surprising to see what you thought was a bomb proof system starting to unravel, but it can then be improved with out much strain. When I'm on the road for real I do the same thing at every rest stop, just for insurance.
I have a good old truck that is pretty much dedicated to canoe camping, I wish I knew of a paint shop that specialized in making a truck look like a leprous piece of junk. Something that no one would covet. In fact so bad looking folks would park far away just on the off chance it was catching!
Best Wishes, Rob

Wire loop attached under hood ...