You have to do several things - assuming you are using an RZ lens, you need to screw a cable release into the lens. Then, trip the main shutter by depressing the shutter button on the lower right, wait for vibrations to cease and then push the cable release to take the shot. You also need to be aware that you have 60 seconds from when you depress the main shutter button to when the shot needs to be completed - thus if you try to take a 15 second exposure but wait 50 seconds before pushing the cable release, you will only get a 10 second exposure since the main shutter resets after 60 seconds. There is no workaround to extend this time, which is why RZ make terrible cameras for long-exposures - removing the battery will just prevent the camera from firing.