Good news - do nothing!
I'm on my 3rd high pressure diesel car now (to be fair first BMW), never even thought about the first one until several hundred miles of driving and someone else pointed out I wasn't treating it like a new car... it was fine. And stayed fine for two years when it was swapped. So treated the next 2 the same way. There is a pretty big clue in that I can't remember the last time I heard anyone buying any new car to run it in, or how.
Somewhere on here a mechanic gave full breakdown of why it doesn't matter these days - I can't remember the techie stuff but it all boiled down to manufacturing processes being so accurate now that there is nothing really to 'wear in'.
Conversely, if you drive any modern engine gently for any extended period you will start to see a drop in performance that is only reclaimed by a much greater period of high rev, sporty driving or lots of well prescribed engine cleaning additives. So really, it's best just to drive the car in the spirit it was conceived. In the case of the 320d, that spirit is surely a mix of low rev cruising and punchy red-line blasts on quieter roads