Which mode?
As well as the speed and direction of movement, the choice of shutter speed is crucial to determining how much motion blur you’ll record. This is why Shutter Priority (marked as Time Value or Tv on some cameras) makes a good mode to work in when you’re experimenting with this technique. It allows you to dial in a specific shutter speed, with the camera selecting an appropriate aperture (and ISO if this is set to Auto) to give what it determines is the correct overall exposure. Manual mode is even better for critical work: you get to lock in all three exposure settings, making it easier to fine-tune the level of brightness.