The settings
Settings will depend conditions outside and on what you want to accomplish. Do you want to freeze the water in time or you want smooth, silky, blurred motion water? Getting a fast exposure is simple, so let’s see how we can get long enough exposure to blur that water:
ISO – Use the lowest possible, 100 or lower if your camera allows. You avoid the noise and it helps with the long exposure we want.
Aperture –To be able to get long exposure and not to burn your photo, you will need to close your aperture. When possible, use the sweet spot of your lens, like f8 or f11. If this is not enough to get exposure you want, and you don’t have ND filter, then close it more, as far as you must to get that shutter speed you want. Have in mind that this can degrade the quality of your photo because of a diffraction.
Exposure – To get that blurred effect it’s good to get exposure of at least 0.5 seconds. More time you keep your shutter open, the blurrier and silkier the water will look. ND filter can help here if the light is harsh, but also a circular polarizer. Either way, this is an artistic choice. Longer exposure means silkier water, but less details in it. So, the choice is on you.
Use the shutter release or self-timer and shoot in RAW for more options in post processing. What I also do is I use exposure bracketing, to get several exposures (normal, under and over exposed) that can afterwards be used in post processing to get better results and well exposed photo overall. If you have a circular polarizer use it to cut down reflections from wet rocks and the water itself.
The composition
This is a big one. You probably had a chance to be in front of the waterfall and it’s impressive, but once you look through your camera it is not looking closely the same. Finding good composition is not easy, sometimes it just doesn’t fit the field of view, sometimes it’s blocked by trees, branches… So, you must put your effort in to it. I use several techniques and there is no universal one, it depends on the waterfall and its surroundings, so you have to try and find the best one.
Working your way up – What this means is that I start from far away and work my way towards the waterfall. That way I also get an interesting foregrounds for my waterfall (creek, rocks, pond…) and I get shots with wider surroundings of the waterfall and a close-up shots of the waterfall. Along the way, you might find your sweet spot.