Use (ND) neutral density filter
Neutral Density filters are normally available in different stops. The type you choose really depends on how long the exposure needs to be and also on the brightness of the day.
It will become nearly impossible to autofocus especially if you are using ND filter that is very dense. Sometimes, it may even be impossible to focus it manually. The reason for this is that ND filter will only allow a certain amount of light to pass through it although your autofocus definitely requires more light for it to function. The same is also true for your eyes. This means that you will first have to remove the ND filter, compose and then manually focus the shot and then finally replace the filter back and shoot. It is not advisable to use autofocus for the step because your camera will attempt to refocus the shots after replacing the filter.
Make use of NDCalc.
With this simple app (available for both Android and iPhone), it is possible to get the right exposure even with the ND filter attached. In order to use this tool, you need to use your camera’s meter to take a reading (without ND filter). After that, inform the app of the exposure time that your camera came got, as well as the type of ND filter that you are using. It will tell you the duration that exposure ought to be.
Take the best photo during the magic hour
The magic hours are times just before sunset and just after sunrise. Even with daytime exposure, it would still be a lot better to take shots during the magic hours. This will give you better contrast against the cloud, something that will make the movements there to be more dramatic.
Have your goals in mind
The weather will not matter at all if you aim to capture the misty look of the water. However, if you intend to capture cloud movement, you need to be more careful in your choice of weather. You require a day that is partly cloudy with decent winds. If a day is fully overcast, forget about getting that beautiful streak appearance as it will be too cloudy. On the other hand, a windless day is likely to offer you only scant sky movement.