No seriously how do you make doorlines
It's hard to explain by words, but I have to open up a small gap between the door and the panels of the car and make some sort of a inside ramp to fill that gap, it sounds simple at first but it requires practice and attention to detail to make it right, the extra time is really worth it if you want to make your car model as polished as possible, so don't rush things up.
There is also the other option of modelling each panel separately sp they can be animated and used with interactive vehicle support, it takes more time, this is not my case but on that side you could ask Barnabas or Teneksi for tips.