They make a spray adhesive special for headliners. I'm getting ready to replace the headliner in my 76 XJ12, so here's what I plan on doing:

Remove the headliner from the car.
Scrape old headliner material from the cardboard backing.
Spray headliner adhesive onto new headliner foam side.
Press headliner into place on headliner backing.

(at least that's how I HOPE it goes!!!)

 

I'm doing my headliner today, so I will post pictures later. Check the interior section for my progress.

 

If you want to DIY, keeping in mind the headliner cardboard is really hard to remove in most cars without bending it, and if it gets bent you will always see the crease. Personally I would contact an independent upholstery shop. In my 96 VDP the Jaguar headliner was like $900 just for the part from Jaguar but I took the car to a local upholstery shop and he did it for like $200 and it looked really good. I have heard some people trying to do this inside the car, but I would think that it would be really hard to get it to stay up while the adhesive dried.

If you do decide to DIY, keep in mind you have to start with new headliner fabric that has the foam pre-attached. You can re-use the cardboard as long as it does not get bent when you remove. You cannot re-use the old fabric. The reason it fell in the first place is the foam is detaching itself from the fabric.

Then you use the spray stuff, lay out the new headliner and let it all dry outside the car. Then you have to get it back in without bending it. Tuck the excess material under the board as you install.

Some guys will tell you on most cars you have to remove the windshield to do it right, but a lot of the shops that do it a lot can get them in and out with windshield intact. I would call around as prices seem to vary quite a bit. I have done real easy cars (Corvette comes to mind) myself with great results but probably wouldn't have tried the Jag.

 

See my post in the INTERIOR section. The removal and reinstall is going off without a hitch.