RULE#1 Time Of Day Is Important
You've decided you want to venture outside (or into the woods) and capture a waterfall. As intimidating as it may seem, it's not altogether that problematic as long as you abide by a few rules.
The first rule you need to adhere to is the time of day. I find the perfect time of day is when the sun is not splitting the trees. On the contrary a dull overcast day is precisely the perfect soft lighting you need. When the sun is splitting the trees you get harsh light and hot spots in your photo and it can really destroy your image. There are exceptions to the rule when photographing in the woods sun splitting images but for waterfalls it's generally not a good idea to go out on a sunny day.
RULE#2 Use The Right Equipment
Get yourself a pair of wellies or waterproof boots. Depending upon the depth of the water I generally use one's that go no higher than the knee so nothing too major. You will need a 10 stop ND filter and also a polarizing filter. The 10 stop filter will force your shutter to stay open longer like the referenced photo above shot at 40 second exposure. The polarizing filter is a good piece of equipment as it will eliminate any existing reflections that are on the rocks including the glare in the water.
Generally speaking with a polarizing filter you must be at a 56 degree angle to your subject for it to work most efficiently. Looking at my photo you can see I got rid of some of the reflection on the rocks but it didn't quite get rid of all of it. Not such a big deal really but if you can cut some of it out why not? Furthermore the polarizing filter adds an extra stop of light giving you additional time where the shutter will be open.
Last but not least is the need for a very good tripod and to be honest I'm using one I bought here for €80.00 or so. It was a manfrotto so you can get away with a more affordable tripod depending upon the weather conditions and the rapid movement of the water. In the woods where I am I've literally got no wind to contend with at all and I'm shielded from it by the large rocks and surrounding trees behind me.
RULE#3 Composition And Camera Settings
I don't get too encumbered by camera settings when it comes to long exposure so I'll just choose the smallest aperture available to me on the lens in this case it being f/22. You may want to shoot at 8, 11 or 16 but it all depends upon what kind of exposure you're looking for and the desired finished image you wish to achieve. I had a large foreground here so I splashed out at f/22 to get as much depth of field as possible and nice silky smooth water. Composition is dependent upon the type of waterfall, stream or nice rapid river that you're currently exploring. I generally try to shoot wide with something in the foreground. I've seen some amazing waterfalls from Iceland where the person photographs themselves or a friend in the foreground in order to give the viewer a sense of the sheer awesomeness and epic size of the waterfall that they're looking at.
Little Splash Of Advice
We only ever get out of photography what we put into it. Sometimes you may see a nice composition but it's covered in debris from the storm the night before. If it's possible then removing this may be your only option to getting the photo but I don't recommend any unsafe practices. If it's a park where many people congregate sadly litter bugs are plentiful among us and so be sure to examine the area for any plastic wrappers or rubbish hiding among the rocks. Sometimes you may be required to get into the water and walk upstream until you find a nice composition. Your enthusiasm in such a strenuous hike into the woods will also determine just how good of a picture you come home with.
I'm sure you may still have some questions so please feel free to ask me anything in the comment section provided below. Best of luck!!!