First, remove front and rear glass, and of course the overhead interior steel trim and upper quarter trim. Remove old headliner, noting which holes the rods were in. Using these is your best starting point later. There should be two wires attaching the rearmost rod to the pinchweld, save them.
You'll need a couple dozen small spring clamps, or a pile of 2" sections of windlace, and some high-quality headliner (contact) cement.
After the headliner has been laid out flat, preferably in the sun, to remove wrinkles and creases, insert the rods exactly as they were in the old headliner. Attach the to rear wires to the headliner rod. Install the headliner assembly in the side rail holes as the old one was installed. Carefully clip the center of the front edge to the pinchweld. This is your last chance to make sure the headliner is OK, so make sure it looks good and will reach the pinchwelds all the way around. Starting in the center front, doing about 6" at a time, using cement only on the top of the pinchweld, cement the headliner across the front pinchweld, using the clamps and/or windlace to hold the headliner. Make sure it's smooth as you go. Now do the same across the rear. Now install the sides, using windlace to hold the headliner in place. It is easier to do this with the roofrail weatherstrip retainer removed, but not essential.
Now install the front and rear glass, using new seals and glass bedding compound, then install the overhead steel interior trim and upper quarter trim.
The coupe is similar, except there is no overhead steel trim, and the seat back and package tray must also be removed.
Now you know why Mustang shops charge $500+ to install a $40 part.
And if anyone tells you he can do it without removing the glass* he's an idiot who will screw up your car, so walk away.
*69-73 does not require removing the windshield, but the rear glass still has to go.