Bulb exposures are a little bit guess-and-by-gosh which is considerably more practical with digital cameras than it was with film where you didn't see the result until the film was developed. In any case, unless you can meter on something and let the camera do its thing, there is a certain amount of trial and error involved in getting the best bulb exposure which often lies outside the normal metering range of your camera anyway. In bulb mode, it just the combination of aperture (including any ND filtering) and shutter time. Usually the aperture is set at an optimum value and several exposure times are tried for the best shot or you can combine several using HDR software, after the fact.
For daytime, long exposures you may need ND filters beyond 10 density. Those are available through photo outlets (e.g., B&H) up to some fairly high densities. The key thing is to obtain a high density filter which is neutral in its color rendition which most designed for ND use are.
If your camera doesn't have the capability for "toggle" bulb (one press starts and second press stops) exposures, you may be able to find an add on (plug in) device which will convert your camera to doing that (plus program long exposures automatically). There are several on the market but finding one compatible for your camera is important so check out what is available and compare it to your camera specifications.
Last edited by Bob 256; 01-22-2018 at 08:28 AM.