Mark the length-wise centerline of the headliner. Chalk will work.

Get a few dull putty knives. Some plastic ones will be good too. You need to pull the window seal down to get to the headliner teeth. Clean any oxidation off the rubber ahead of time. Make sure the car is out in the sun to make the seal a little more supple too. Heat gun will help and for other things.

Get clothespins or wide paper clips to later hold the edges of the headliner glued to the side rail.

Let the headliner sit out in the sun.

I install the screws for the mirror and sunvisor ahead of time. After you install the headliner use a 1/4 hex socket just smaller than the screw heads and burnish perfect circular holes. If you cut any holes at an angle it will rip.

Start installing from the center outward, using your centerline.

Use the best glue you can get your hands on.