When you take the headliner off you will find that the glue is covered in a thin layer of foam that has deteriorated. The foam on the headliner material can be turned to dust practically by rolling it in your fingers.
The headliner shell has to have this dust that is stuck to the glue removed before replacing the headliner with new material.
I used acetone and a scraper outside the garage.
Follow the instructions on the glue spray can, some say you should use 3M Super 77 but I found Permatex Heavy Duty Spray Adhesive to be just fine.
Do not stretch the new liner material too much as it will tear.