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.