The headliner is held snug around the rear window by a long (like a couple meters long) spring. On each end of the spring are hooks that attach to the hat shelf, with the length of the spring running through the seam in the headliner itself. That's the only part of the headliner that doesn't get any excess tucked into the edges, so it's easiest (best?) to start there and work your way toward the front. The windows won't be in your way.

Best,

Cameron
Rose City