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.