IIRCC When I did it I took the door trim off but as its all only accessible through the speaker hole I think that was unnecessary.
So take the speaker out.
Stick your hand through the speaker hole in the door and ***** around the bottom of the door, you should find a loose piece of plastic strip about a foot long laying in the bottom.
Pull that out, figure out which way round it goes and with the window up slot it back into the inside of the window runner that runs up at an angle at the front of the door. Once its in and with your hand well clear check the operation of the window.
Put the speaker back and enjoy rattle free motoring, until next time.