I too am to the headliner stage, and I will share the secret of the trim clips. I salvaged mine from a junkyard and had to figure out how to remove them without breaking them, lest I have to pay for new ones. So, what I did was to take a small flat bladed screwdriver and work them out SLOWLY. The rears splay out to the sides and are stepped where they go in. To get them out, you need to push the sides in while applying gentle pressure downwards, which you will be doing anyhow to be able to see them. The clips will walk down the "stairs" a step at a time as you gradually do one side, then the other. The fronts have a clip that moves front to back and not side to side. To get those, you need to either push or pull the center tab. Work slowly, and you can get them all out without breaking one. The sides are by far the easiest. You just push on them from the inside to the outside of the car and they will unhook at the outer edge. Then, they just come out once you slide them out of the A pillar molding. You may wish to practice on a few junkyard cars first, just to get the hang of it. Plus, if you succeed in the junkyard, you will have spare clips to do your car.
To reupholster the headliner board, you need to first clean off all the old foam from the shell. I use a small scrub brush and a shop vac for this. Just work gently so as not to damage it. Now, lay out your headliner fabric on the board and make sure you have a lot of overlap over all the edges. Next, fold the material in half. You will now need to coat BOTH the material and the shell with 3M General Purpose Upholstery Adhesive. DO NOT BUY DURO BRAND!!! It will fall in 24 hours. Once the glue becomes tacky, put your hands inside the center of the fold and work out to the edges. You will gradually unroll the material onto the board this way. DO NOT just flop it over all at once unless you want to ruin it. After the first half is done, you can fold back the other half and repeat the procedure. Once you are done, wrap about an inch or two of the excess around the back of the board and glue it there.
As for what to put above the headliner, I will share what I did with my truck. I used some cheap imitation Dynamat off E-Bay and cut it in strips. I then put the strips on the roof, spacing them apart. This helps cancel the vibration waves in the metal while using less material. As for insulation, I used some of that rebonded jute stuff (fiber scraps like in a G body door) that has a metallic backing. I cut it to size and put it on top of the headliner and then put the headliner back up. It isn't glued on in my truck as I didn't think it necessary. I also suggest replacing all of the old sound deadening jute in your car if you gut it. This stuff usually holds water and mold, and over time contributes to the "old car smell". Just don't do it if your car still has any water leaks or it will be wasted money.
1985 Cutlass Supreme Brougham (ex pizza delivery beater with 240,000 miles):
355 Chevy- 9.5:1 compression, Comp XE 262 cam, World S/R Torquer heads with 2.02's, Edelbrock Performer, 800CFM Q jet, Camaro ram air system,Edelbrock headers, Comp Magnum roller tip rockers,TH 200 4R, 3.23 rear, aluminum hood, IROC steering box, poly bushings, Eibach springs, etc. Cutlass build thread here: viewtopic.php?t=4982
Also: The Thing That Won't Die-1998 Nissan Frontier, 340,000 miles of pizza delivery.