Getting exposure right when using ND filters can be tricky, unless you enlist the help of NDFilter. This open source app for Android makes it possible to calculate the correct shutter speed for the given ND filter with consummate ease.
Switch your camera to Manual or Shutter-priority mode and set the correct exposure without the filter. Enter the obtained shutter speed value in the app, select the desired filter, and NDFilter calculates the appropriate shutter speed. All you have to do then is to attach the filter to the lens and set the camera’s shutter speed to the calculated value.