First you set your aperture and shutter speed as you want your environmental lighting to be. Just use the camera normally in M mode.
After you know your exposure time and aperture (and of course ISO), let's say you got your environmental exposure as 4 seconds, f/3.5 and ISO 400. Then you scroll exposure to Live Composite mode, hit Menu and choose the exposure time that you got, that is the 4 seconds.
Place the camera on tripod, frame your photo and release shutter once to let camera meter the scenery for preliminary frame (that is the 4 seconds with f/3.5 and ISO 400 you chosed) and after 4 seconds you are ready to start stacking photos just by a pressing shutter release button again. Sit down, enjoy from the fireworks and watch the...