A tripod will bring best results, but a steady hand or solid camera rest will be ok for practicing (eg, just shooting a water spicket or rain outside).
For practicing, use cheap ISO100 film. For digital, set ISO as low as your camera allows.
Step by Step:
- Set up tripod
- Compose the scene
- Turn the mode dial to Time Exposure (Tv on a Canon).
- Looking thru the viewfinding, adjust the time exposure to lsomething slow. (Note: The faster the water is moving, the quicker it will blur. Curt said 1/8 of a second...sounds good to me for a waterfall.
- Take the shot.
Remember, it's just moving water. It's like trying to blur anything moving. The faster it moves, the less time required to blur. The slower it moves, the more time.