What sort of camera are you using?
First of all you'll have to use a tripod, or something similar to mount the camera.
Also, it's worth using a cable release or the cameras own self timer to start the shot so you don't blur the shot by pressing the shutter manually.
A light metre would probably enable you to get an accurate reading, but I've never used one personally, I've always just used the metres built into the camera. You'll need a camera that can take a long exposure, either by using presets (eg 30 seconds), or one where you can use a cable release to lock the shutter open manually for the required time.
Obviously if you're using digital then you can just mess about until you get a good result, but if you're using film then you're going to have to experiment to a certain extent, unless your cam can match the metre readings.
Also, it's probably worth using a fast film.