Geez guys the guy is only asking a simple question.

To the OP: Just put the camera on a tripod, set the camera on B, choose f4 or f5.6 to start with. Use a soft-release with a lock capability.
Click the shutter and keep it open for an hour.
Then Two, just to have a more pronounced effect.
And then 3 or 4 hours, if you have the time to do so.

That's it.