It’s not really difficult to do, but you do need some of the right equipment to carry it out successfully.
To achieve that look, your camera needs to get down to shutter speeds as low as ¼, 1/3, 1/6 of a second and possibly slower and you will also need a good sturdy tripod to give your camera a solid base.
If the available light does not permit you to get down to those low speeds, and you are using a SLR camera, you could get by with a Neutral Density (ND) filter. There are various grades available to suit different situations. You may also use a polarizing filter. Slowly rotate it until you get the desired effect. Polarizers can also drop the speed and lower the light by up to a couple of stops. As a last resort, find some way of attaching your sunglasses to the front of your lens. Desperate means - desperate measures!!!
So, after reading the above, and you still feel confident about the situation, there are some exposure difficulties that could arise and do need to be addressed. That is, that rushing water can and does reflect a lot of light and if the scene consists of dark areas as well, the camera could expose for these dark areas and the water will take on a washed out appearance lacking in detail. On the other hand, it could expose for the bright water and the rest of the scene will be in darkness. Assess the brightness levels in the scene and if you think there are some extremes and you have a camera that allows you to operate it manually, take a light meter reading of something in the scene that is a neutral colour. Such as a grey rock, a patch of green foliage or even Caucasian skin colour will give a neutral tone. Take a note of that reading (shutter speed and aperture setting) and set it manually into your camera. This should provide for a better shot, but don’t rely on that, get lots of shots at various settings, till you feel you have it right.
See "Altering the Camera's exposure Settings"
If it is a waterfall you are photographing, make sure you take ample shots at different angles and viewpoints. If you have to return at another date and time, it may have all dried up.