First things first. I started by collecting blueprint images of the E46 from the net. The red and black lines were drawn to align the views and guarantee they are resized to the same scale.
These blueprints were then set as background images on the blender project:
Using these background images and several pictures of the vehicle, the main lines were sketched in 3d:
Sometimes I had to use some tape on the real model and check if the curves were right:
After a few iterations I did a test render of the model and a turn plate animation to check the overall look of the curves.