You'll need to pull the headliner to expose the nuts/clips that hold them on there. The headliner isn't too hard to remove (lots of tacks and clips, etc).

Start with the trim piece that is on the back of the headliner between the tailgate hinges. There are some phillips screws there. Once you get started, you will see what the next step in getting it all out is. Take notes and don't lose any little pieces.

May I ask why you are taking the roof rack strips off? Detailing?