I am going to try to cover some of the basics of long exposures - stuff I learned from individuals, things online and items from my personal practice in this type of photography.
A Neutral Density (ND) filter is a must. You may be able to get by with a Circular Polarizer but the ND works better and produces nicer results. I carry several densities, 0.6, 1.2 and a Hoya NDx400. As I have been trying this technique more, I notice that I could really use 2.1. If I had to do it over again, I would get a 1.5 instead of the 1.2, but that is just a personal preference (I have noticed the slight desire for a bit longer exposure on some waterfalls where the 1.2 was just not enought).
Make sure you keep the filters clean at all times. I have recently been going through my photos and noticed many that had spots from dirty filters on them - I now ALWAYS carry at least two lens cloths and one LensPen with me. I check the filter after every shot when I am around waterfalls (the falls produce a wind that carries mist and constant attention is needed) and also shooting other subjects. Nothing is more frustrating than a dirt spot that cannot be cloned out in post-processing, ruining a great photo.
When using a dark ND filter, you will definitely need Live View if your camera has it. This makes framing the picture so much easier than using the optical viewfinder in DSLRs. I am able to frame with a 0.6 ND filter with no issue. The 1.2 is a little difficult - especially in shady forests on overcast days - Live View is used in this instance. With the NDx400 - no question - Live View only. Using the viewfinder with this filter is a no-go as all that is seen is blackness - Same as if I would have the lens cap on and trying to get a photo.
When taking photos with long exposures - especially those of 20 seconds or more (shorter if in bright light) - the viewfinder needs to be covered. I carry the viewfinder cover and a spare with my gear at all times. I read about and experienced light-leaks with exposures in bright sun. The photos have definite red lines in them - can't do anything with these except delete them. Covering the viewfinder fixes this issue. If you don't have the viewfinder cover, a dark cloth can be used. It is just so much easier to use the cover. Chances are - B&H Photo has the proper cover in-stock for your camera.
A sturdy tripod is a must!!! This is required for sharp photos. Also - remember to turn-off any shake-reduction in the camera or the lens. The mirror being locked up also helps (but Live View will do this for you). What I found through reading and experience is that the camera's built in 2-second timer locks the mirror and gives me time to be away from the camera and for it to settle before exposing the image. I also use a cable-release when taking these photos. I want to be as certain as possible that there is no movement.
All of this takes practice. I am not at the level I want to be with my long exposures but I can say that I have definitely improved my skills at capturing these images. Check out "Long Exposures" and "NC Waterfalls" galleries on my site, these are examples of what I described above.
Until later - Happy Photo!!