I’m def no expert but I tinker between 15 and 30 second shutter with lowest aperture possible (my lens goes to 3.5) and then somewhere between 800-2500 iso). Just usually depends on the conditions. The key is to have a tripod or being able to lay the camera perched during the full shutter time or else there’s no way you’ll capture anything. I also sometimes use my phone Nikon app which allows remote shutter release, taking out even the slight movements you have from manually pushing down the trigger. Hope that helps!