Sorry to say, but Beginner is right. There is no simple way to do it right. There are solutions out there...but, to do it right means pulling the interior cabinets and partitions....then the headliner...then completely clean off all the previous adhesive residue...then re-adhere a new or thoroughly clean liner again. I always ask about the headliner condition in my search for a 90's 21' Sovereign
. When I find one, the liner is already hanging or the seller not willing to understand the very expensive problem.