Set the camera securely on the tripod & attach your cable release.
Set the camera securely on the tripod making sure that the legs and head are all secure and movement free! If it’s windy, consider using your bag to weigh the tripod down slightly.
The last thing you want is to ruin your lovely long exposure with a load of tripod movement. Plug in your camera remote and use this in lieu of the shutter button on the camera. Triggertrap Mobile is awesome for water smoothing photography, as the different long exposure modes make setting the exposure time really, really easy.
You can use the Timed Release mode and input the exact duration of your long exposure.
Set your focus.
Focus the camera, either manually or using autofocus, then switch autofocus off. Keeping the camera set to manual focus will make sure the focus doesn't shift when you hit the button to start your exposure.
Attach the ND filter to your lens
Carefully screw the ND filter (or filter holder) onto the filter threads on the lens, making sure the filter is nice and clean! If the front filter thread rotates, double check your focus after you’ve added the filter. This can be tricky to do, so live view might help you out!
Set the exposure
Put the camera on manual with the slowest shutter speed possible, whilst retaining a ‘good’ exposure.
Set your ISO to the lowest possible number and the aperture to the smallest possible aperture (e.g f/22), then, using the exposure meter, set the shutter speed so that the exposure is correct. if this is greater than 30 seconds, use bulb mode and your remote cable.
You can try and guess the exposure, then use your screen and histogram to determine the correct exposure through trial and error. Alternatively, get the correct exposure for the scene before adding the ND filter, and then calculate the correct exposure by adding on the number of stops that the filter adds.