First install only the headliner, push it up (a centimeter max) and use a pencil to draw a line in the hole.
Then you make a template form a piece of carboard or something and tape it exactly in the markings you made before. Then drill three holes in a triangular shape.
Use three screws to bolt down the template on top of the car. This way the template is exactly at the same place as on the inside. Success guaranteed