You wont need to actually drop the whole headlining totally , but if you do - its straight forward.
In the area you need access to - remove the necessary trim furniture from the headlining ( grab handles , sunvisor ) as applicable.
Then remove the required aperture seal ( front door , rear door , tailgate ) as applicable.
Then gently and slowly pull the headliner down to the point that it looks/feels like it will crease ( its like thin hardboard or cardboard) and place your hand between headliner and roof panel.
If there is not enough room - take more off the car per the points above.
Just be gentle and keep your hands very clean- ok