For the D2 its easy !.
The bars come with a drilling guide that has a shoulder on it that locates in the bolt hole in the roof re-enforcing inside the car.
You drill through this from the inside with a 4mm ? drill, then down from the top with a 30 odd mm hole saw. Paint the bare metal edges.
Theres 2 bungs in the front rails rear roof mount, hook these out as this is where a dowel goes in the front of the extended rails.
Del