Hi
You need to be able to slow the shutter speed down to achieve this, so camera on M or Tv mode - and you just adjust the shutter speed downward until you get the desired effect.
There is no correct shutter speed, as each shot will be different.
You really need a tripod or means of holding the camera rock steady so that the image doesnt blur due to camera shake, yet the light trail remain as true as they can be.
Andy