It's really quite easy and you just need a few items apart from your camera. Most importantly you need a stable surface or a tripod. If the light is just right and dark enough you might be able to do it just with the tripod but more than likely you'll also need an ND (neutral density) filter which is basically just like sunglasses for your lens. A polarizer filter is also a good idea to cut down on the reflections from the water, rocks and leaves. One other item which is always good to have but not essential for this is a cable release which allows you to trigger the camera without actually touching it.
With an ND filter on the lens you'll be able to set a slower shutter speed which is what gives you that silky look. Basically anything slower than around 10th/second should give you a nice effect but experiment with slower shutters down to 1 second or so for a very nice smooth effect.