When you’re ready to go, simply hit the button on your cable release or Triggertrap Mobile and wait for the exposure to finish.
With any luck, the water in your images should look lovely and smooth!
If it’s not smooth enough for you, you might want to consider stopping down as much as possible (using the smallest possible aperture). If that’s still no good, then you may want to think about purchasing a more dense ND filter. There are a couple which go up to 10 stops – however these are considerably more expensive than their less dense counterparts.