I've never restored one, but I had the one out of the green car a couple times. It's not a bad job.
Pull at least one front seat, but both make it easier.
Remove the back seat. (bottom, and back)
remove visors, trim, (including those big bits around the rear windows,) Map lights, and the rubber trim-ring around the sunroof opening (if equipped.)
There's at least one "mushroom" fastener somewhere. (rear middle?)
After that, you should be able to drop the headliner, and maneuver it out one of the door openings.
It's a more or less rigid piece. I would think it is difficult to replace / restore the fabric independent from the rest of it, but I've never encountered one with the "sag." That might make it easier if that's already happening.