Simply follow the dozens of headliner replacement posts across all models. They are all the same procedure. Remove any trim, visors, handles etc holding up the headliner backer board. Strip the board free of the foam backing the head liner material. Using appropriate spray adhesive spray 6-8 inches at a time and smooth your new material onto the backer board. Reinstall backer board and trim.
As RRToadHall states plenty of write ups if you want to DIY. I will add the following though 1) Do the sunroof 2) Depending on how bad your current headliner is be careful of the crumbling foam/glue during removal as it is tough to get out of the carpets. (The hardest part is removing the old material)
I did mine myself and it doesn't look too good, but at least it is not rubbing in my hair anymore. As I did it I see all the mistakes, but my wife and kids know no better. The RR is trickier than many other cars due to additional curves and size. I procured my headliner from Joanne Fabrics, they had two colors in stock - gray and tan. Mine was gray and looks fine with the pillars which I did not do - but not a perfect match. Call around and ask local fabric stores if they have headliner material - many do.
The price Keralis received is awesome. With material and glue I think I had about $80 in material - I think I sourced my spray glue from Wal-Mart. If you do not want to do the actual application yourself maybe call around and see if some shop will do it if you bring in the headliner ready to cover and the material or some combination.