Just follow this instructable...
Step 2: Camera Setup
You'll need to be able to manually set the exposure (shutter speed), Aperture (the f-stop), and ISO.
Depending on your camera and how close you are to the fireworks, you will need to adjust the f-stop to fine tune the pictures.
Longer exposures will capture multiple fireworks bursts. 2 to 4 second exposures work great.
Step 3: ISO
You'll want to use a low ISO value, around 80 - 200. Although most cameras can shoot at higher ISO's such as 1000, the images will have more 'noise' and look 'grainy'.
I used an ISO of 125 for the photos in this instructable.
Pick a low ISO and stick with that. If you find that the photo is underexposed, adjust the aperture.
Step 4: Aperture
The larger the f-stop number, the smaller the iris is open.
What aperture to use depends on your camera, and how close you are to the fireworks.
You will most likely need to adjust this setting. Take some shots at around f-8 and if they look to be underexposed use a lower value. Overexposed then use a higher value.
The photo below was at f-11 and is under-exposed.
I found that f-5 worked for me.