Tripod and Lens Selection
To get the full effect of the exploding fireworks, you need a long shutter speed, anywhere from half a second up to two or three seconds. When you are shooting at those speeds, handholding your Nikon D300 is not an option if you want quality results. Attach the camera to a tripod, and install a standard lens 50mm lens to the camera. Depending on how close you are to where the fireworks will be launched, you may want to use a wider lens if you are very close, or a longer telephoto if you are farther away.
Shutter Speed
You will want to capture the full effect of each individual firework burst. To do this, you will need to use a longer shutter speed. Start at one second on your shutter speed dial and see how much of the firework burst you collect on your image. If you decide to increase the amount of time, remember to decrease the aperture. If your starting aperture is f5.6 then go to f8.
Aperture and ISO
Set your Nikon D300 ISO to 200. You will want to use a lower ISO to allow you to keep the shutter open for as long as you will need. With a lower ISO you will also minimize the noise that long digital exposures create. Set your aperture at f5.6 to start. You may need to adjust the aperture after the first exposure, and generally this is a good place to start. It is in the middle of the aperture range and will give you a slight increase in depth of field.
Manual Focus and Review
Auto focus is too slow for fireworks. Once your camera is set on the tripod, focus the lens to infinity. When the first firework goes off, review your settings and make any adjustments you need.