For the one piece TMI headliner, they say you do not need the front or rear windshield out but to put the sail panels in properly, you do at least need the rear windshield out. Also, the package tray trim on both sides should be removed. Here is how I installed mine, it wasn't that hard of a job but it would be easier with a helper. I removed the headliner side and front trim. I got 4 threaded rods that fit the dome light threads and put those in place. You may want to add a small hole for the dome light wire to pass through, I did not but ended up having to add one when I added my dome light later on. I had to turn the headliner sideways to get into the car then I held it up and used the 4 dome light threaded rods to center the headliner. I used a bunch of washers as spacers and put nuts on the threaded rod to hold the headliner in place. Once it was in place, I installed the front trim piece, then the side trim to include the bolts for the seat belts. I did not install the rear side window trim pieces (short ones) at this time because the sail panels go under these. I installed the sail panels (the retainer piece on the back does not line up, don't bother with that. I pulled the part of the sail panel out of the rear window that goes under the window and glued that down to hold it in place then set the window with new gasket installed. Used a rope to get the window gasket in place and to wrap around the rear of the headliner (rear of the headliner will have the window gasket around it to hold it in place). After the sail panels were in place, I put the package tray trim and the rear quarter window trim on.
I hope this helps, let me know if you have any questions or need clarification. Good luck.
Tom