Really long exposures, in the order of a quarter of a minute up to several minutes, need strong ND filters of several-to-many f-stops. Most filter manufacturers produce filters in a variety of strengths, and a set of three or four will give you flexibility, enabling you to combine two or more as necessary. Here’s what you need for long exposures:
A set of ND filters ranging from around three stops to about nine stops will enable you cope with most daylight situations.
Tripod (needs no further comment).
Always work at your lowest ISO.
So as not to waste time, it helps to have a rough idea of which strength of filter to reach for first. As a rule of thumb you can start with that old-fashioned but still relevant ‘sunny 16’ rule, which says that if it’s sunny, set the aperture to f/16 and the shutter speed to the reciprocal of the ISO – so 1/100 sec at ISO100. In this example, you’d need a seven-stop filter to get you an exposure of a second (1/100>1/60>1/30>1/15>1/8>1/4>1/2>1), and an 11-stop filter to reach a quarter of a minute. Stopping down to f/22, as you can with most lenses, doubles the possible exposure time yet again.
Focus and compose before you fit the filter(s), because once they’re attached you won’t be able to see a thing. Oh, and disable autofocus, too.
ND filters are often available as kits, which can save you money compared to buying them separately