There is a kit that comes with the foam blocks, two ropes, and a "Volkswagon" strap.
I call it the "Volkswagon" strap, because there was a VW commercial where some guys were trying to load a Golf with household stuff during the duration of a traffic light while in line for the light. They successfully load it, including tying a matress to the roof. After they had the matress tied on, they rejoiced because they had beaten the traffic light. However, when they went to open the front doors, they discovered that they had tied the rope holding the matress to the roof through the windows of the front doors; making it impossible to open them. The commercial fades to black as the VW crew gets heckled, New York style, by the drivers waiting with them for the traffic light.
Any way, back to the canoe. Put the foam nerds on the gunnels, lift the canoe and center it on the roof of the car. Adjust the foam nerds so they are on 'hard' parts of the roof.
Open both front doors, verify that all is clear and the toss the buckle end of the "Volkswagon" strap over the top of the canoe. Pass it into the passenger compartment, pass the bitter end through the buckle and snug the strap down tightly. I useally put the strap to the forward part of the door opening as I'm 6' 2" and I don't want to have my head bumping/rubbing on the strap.
Add a bow and sturn line, tied in an inverted 'v' to the bumper shocks/brackets and your ready to go.
I usually tie a bowline knot to one of the bumper shocks, run the bitter end though the bow/stern of the canoe, go down around the opposite bumper shock/bracket, back up to the canoe, and back through what ever the rope went through previously. I then take about ten tight turns, hangman noose style, around the apex of the inverted "V" and then do two half hitches. This hold extremely well. No issues at interstate highway speeds either.
And you can see both of your bow and sturn knots, and monitor the tension of your "Volkswagon" strap. If anythin starts to loosen (it won't), you can stop and correct the issue.
I have a Thule rack for my rabbits now, but I still use the "Volkswagon" strap, and the bow and stern lines, and it still works great.