If I remember correctly, the headliner just tucks into the channels at the edge using a dull thin plastic object such as a plastic putty knife...but make sure that it's dull or it will cut the fabric. Those rods in the headliner...don't throw them away. You'll be reusing them again. Start from the front and work towards the rear. In that way any minor wrinkles left over will be out of sight. Oh...come to think of it, you'll have to remove the windows because the headliner glues into the rear and side window frames. Almost forgot about that. It's been 35 years since I worked at a VW dealership!
The windows may be damned near impossible to replace now because the rubber molding by now is very hard from age. If you can get new rubbers from a supplier somewhere, a thin nylon cord and silicone spray makes replacing the windows easy. Spray the rubber, wrap the nylon cord around the rubber molding one complete turn, have a second person hold the window into position over the metal frame and use just a little bit of force pushing on the glass while you pull the cord out from inside the car. It's that easy IF the rubber isn't hard.
*Hey, are you aware that you can put 1500 cc pistons and cylinders on that 1300 cc engine? >>>I think that you've got the 1300 cc engine, or is it 1200 cc?