Like dolfan mentioned and link is a detailed pictorial on the headliner install on my 72 coupe. The procedures are pretty much the same for you except that you have 2 thin wires that put tension on the rear bow. But it's essentially the same. I believe on 67-68's you can even leave the front window in, but I highly recommend removign teh rear window. If you had a 65-66 you would have to remove the front windshield also to install it correctly.
It takes a lil time to do but it's fairly easy. Just be very patient and check twice before trimming back the faric listings.
I got a question though, what is the best way to install the front and rear windows after the headliner is glued to the lip, Ive always seen/used the kite string trick but that seems like it would pull the headliner off the lip.
I haven't redone the front glass of my 65 but I had a shop replace the windshield on my 72. On the rear window installation i put the weatherstrip on the window (with glue in that channel) and then put glue in the outside channel that the body goes into. Then i set the bottom of the window in the pinchweld then worked the weatherstrip around carefully pulling the lip over the pinch weld with a wide slotted screw driver. I tried the rop/wire method and it didn't work really well with me.
I used just the general 3m adhesive and not the "super" kind. I used it casue I had it and it said it could be used for headliners on the bottle. SLight problem, its not sticking worth anything. Will "super" fix this problem?
I highly recommend the tried and true method. Take it to an upholstery shop.
Of all the "do-it-yerself" stuff I have been told that the headliner is by far a no-brainer. Unless you only make 5 bucks an hour it just does not pay to do it yourself.