The headliner is glued in place with contact cement. I bought some from 3M off their web site along with some special solvent for removing old the adhesive.
I bought a headliner from OGTS. I planned to install it myself. After a trial fit, I wasn't convinced I could do it without screwing it up. So, I ended up paying an upholstry shop $100 to install it. Some of the other quotes I received were as high as $300 just to install.
The deal I struck included me removing the glass and any of the interior I did want them to screw up. I already had the new windshield rubber, so I cut the old rubber to get the glass out.
After talking to the shop owner, The trick is to use REAL STRONG contact cement, spray it on both of the mating surfaces, steel and fabric, and let dry completely. Then reinstall the headliner and stretch and attach the headliner to the steel. The cement has to be strong enough to hold the fabric in place to the steel as its being pulled repeatedly tighter as one works from the rear to the front. Pictures are in this thread...
It was worth it to me for someone else to do it once the right way.