Recently did this job (successfully). You can ignore the part about peeling off the old lining material from the shell.
The headliner on my 1986 GMC Sierra Classic looks like factory. Here's how I did it:
1. The molded backing board/shell (don't quote me on nomenclature) is the key to this whole repair and it cannot be in the vehicle to do this properly. Remove the pieces that support the headliner.
2. Peel the old headliner fabric (if it is still there) carefully away from the shell. If any pieces of the shell start to come off with the fabric separate them with a butter knife and set them aside. You can glue them back into their original positions with adhesive cement.
3. Step back and see what you have to work with. If you are lucky the fabric will have cleanly separated from the fiberglass shell. If not, that is why you so carefully saved those little chunks. Glue them back into place and let dry.
4.Get a shop-vac and a brand new toilet brush (toilet brush - not a wire brush). Get down on your hands and knees and start firmly - but gently - removing all of the failed adhesive. Don't worry if some of the shell carries away - it is actually better to get down to a new surface. After you are done scrubbing use the soft brush attachment for the vacuum and get all that old glue off. Then get your toilet brush back in hand and do it all over again, and again until you have a new surface to work with.
5. Go to the Home Depot and get a couple of cans of spray-on polyurethane and 4 cans of 3M Heavy Duty Adhesive - don't substitute on this. Oh yeah, also buy a paint roller and a nice fluffy cartridge.
6. Spray the newly cleaned shell with the polyurethane and let it dry overnight. In the AM you should find that you have a nice hard/firm/sturdy shell to work with.
7. I forgot to tell you that on the way home from Home Depot you should have gone to JoAnn Fabrics (plenty of stores in Colorado) and asked one of the ladies there to direct you to the headliner material. You also should have measured your shell or brought your old headliner fabric with you in a plastic bag. They at JA Fabrics will set you up a beautiful piece of foam backed headliner that is thick enough to hide any gouges you may have made during the cleaning process.
8. Back down on your hands and knees. Lay the new material out over the shell and trim to fit (leaving about two inches over on all sides.
9. You can guess the rest - read the instructions on the 3M adhesive can. I believe it will say to spray both surfaces and wait 30 minutes before joining. Use all 4 cans - it seems like a lot but trust me! When you are done spraying, it will look like a winter wonderland, but that's correct.
10. It really makes things go better if you have an assistant to help you when you go to join the material to the shell. Lay the fabric back down on the shell and get that new paint roller that I mentioned earlier. Get back down on your H&K's again. The, while gently tugging where needed, start rolling the fabric onto the shell. Work from the middle outward. If you get any wrinkles, don't stress, you will have time to lift up and relay the material.
11. Let it dry, do the final trimming of excess fabric and go hang in back up in the car.
One note; depending on the contours of your shell it may be prudent to place some folded up towels under certain areas of the shell so that when you roll the glue into the interface you can press down firmly.
This will work and you will not be able to take your eyes off of the wonderful job you did ( and keep showing people till they are sick of hearing about it). It will last for 20 years.
