You need the roof rails themselves, as well as the roof-side moulding. Infiniti considers the moulding to be non-reusable and advises to "always replace after every disassembly". So you may want to buy those new, from the dealer, even if you go the used route for the rails.
There are approximately 5 clips holding your non-roof-rail moulding in place. You can use a trim removal tool to work the moulding up and detach it from the roof. Start from the back and work your way forward.
The roof-rail-moulding has 4 clips and a pawl that holds it in place. Put the pawl in the correct place (front) and snap the clips into the roof.
To install the rails, you must remove the headliner so you can access the inside of the roof to attach/tighten the 3 nuts that hold the rails in place.
Removing the headliner is quite involved.
- You must remove the light & rain sensor cover.
- Then remove the front and rear body side welts.
- Remove the front pillar garnish and disconnect wire harnesses.
- Remove center pillar upper garnish.
- Remove rear pillar garnish and disconnect wire harness.
- Remove all assist grips from roof.
- Remove overhead control panel control unit.
- Remove sun visor assemblies.
- Disengage headliner assembly pawls at front and clips at rear.
- Detach headliner from magnets on roof and remove through rear hatch.
Once the headliner is removed, you can put the rails in place and tighten the three nuts from inside the vehicle.
A better option, imo:
If you don't have the factory roof rails, I think it may be better to buy the Thule Kit 183117 and then you can choose either the Wingbars Evo 118 or AeroBlade Edge 7601+7602, depending on which crossbar style you like.
The Thule options can be installed without dropping the headliner and both of them look good (although I think the AeroBlade Edge looks better than the Wingbars Evo)