I don't know if a '51 is different, but on my '54 the front end of the front piece of cardboard just wedges in behind the windshield rubber. The winshield rubber sort of has a flap that you can roll back to fit in the cardboard. It works the same way over the door openings, also the middle bow tucks in behind the door rubber. Make sure you push the back edge of the front piece of cardboard into the middle bow 1st and crimp it. This will give the headliner the shape it needs to assume for installation prior to actually trying to fit it into the truck. Good luck, I just did the job last night with only one little wrinkle. :rolleyes: