Tips to install cardboard headliner?

If the windshield and rear windows are out, that's a good thing. As JR said, the headliner goes under their rubber seals. It's easier to install if you don't have that to worry about. You just have to be careful with the fragile headliner when you install the window seals so you don't scratch or chip it.

There is probably a round button with a spring clip on its back that is holding up the headliner at the very back, just above the rear window. If your rearview mirror is mounted above the glass (not glued to the glass), then it has three screws into the roof framing, and that will hold up the front of the headliner. The sides have two or three small metal clips that hold the edges of the headliner to the roof rail pinch weld. The edges, of course, are covered up by the textured metal trim strips. Finally, the dome light holds up the center.

You'll have more room to manuever the headliner into position and prevent damage if you first remove the quarter (pop-out) windows. In fact, I don't really think you'll get it in there otherwise.

Good luck,
Jerry