Well, this is three weeks later, and maybe you're done by now. I've cut, sewn and installed around a half dozen PV headliners over the years, and here is my 2 cents.

First, there are differences between the PV444 and the PV544 headliners, the most obvious being that one is cloth (444), and the later ones are perforated vinyl. Also, the PV444 has a loop sewn into the rear window area of the headliner which can be pulled tight from inside the trunk. A lot easier to install one of these, trust me. I think that US cars of the era had a similar method.

Second; The 544 has a series of black plastic strips sewn into the area around the rear window, which fold over and "slot" into metal channels at the edge of the opening. Sometimes there are tiny metal clips which secure the headliner at the critical corners. A "repro" headliner should have those strips already sewn in place, but, as I said, I've always made my own (VW material is darned close in appearance, Mercedes stuff, too.) Having the rear window out will definitely make things easier, also when using carpet tacks to secure the lower edges of the headliner to the package shelf. I have glued the rear sides to the body (around the rear window), but it's not so easy not to make a mess of it. The factory didn't use any glue, so it was basically a mechanical installation.

Third; and IMHO the most important, I've always started from the FRONT, and worked my way back to the rear window, then finished by tucking and trimming along the sides and front. There should be a circa 10 inch sheet metal strip up near the dome light. Use that to secure the first metal bow in position. Fabricate a securing devise (coat hanger wire, for example)if that strip is missing, it keeps everything rigid as you pull towards the back. In some cars there are small clamps, or clips on the first bow to keep it upright. The bows are marked with numbers I through 5, and it makes a difference how they are installed. Use some kind of padding (non moisture absorbent) between the bows and the metal roof. It prevents squeaking. Pull the material slowly, allowing it to stretch gradually before cutting anything away. Leave at least 4 to 6 inches of excess on all sides, before doing the final trimming. Remember to mark carefully for the dome light, and DO NOT use a Sharpie, or ball point, or anything but PENCIL on the fabric. It WILL bleed through. Been there, done that.

I've always bought 2.5 to 3 yards of material, just to be on the safe side.
Anyway, my headliners have been looking better and better, so it can be done.
I would have LOVED to witness the Swedish guys on the Volvo assembly line, back in the day. It probably took them less than 5 minutes to install a headliner. WOW. Takes me about 1.5 to 2 hours, and for a newbie, plan on a long, long afternoon. Lay the new headliner in the sun for a while, to get it to stretch out first.
Good luck
BTW Porsche and VW headliners are a F***** nightmare, so you're getting off easy!