I usually try to pick a location as close to the action as possible. In addition, find a spot that is free from obstructions. You don't want people walking in front of you when you're trying to take a great photo.
The fireworks will be too far away for your flash to make any difference in the image. Be sure to turn it off.
Night photos with smartphone cameras are usually a little grainy. Do not zoom with your iPhone. It will just make the graininess worse and more out of focus.
Your camera will have a hard enough time trying to capture photos at night. Help it out as much as you can by stabilizing it. I usually lean it up against a tree, a chair, or a park bench. If it's not too much of a distraction, I'll even lay on the ground to get a great shot.
Most people know that they can tap the screen to focus on a particular part of a photo. If you CLICK and HOLD, your iPhone will lock the focus AND exposure until you snap the picture. This will ensure that your fireworks are clear and keep your camera from trying to focus on something else.
Most people will try to touch the shutter button on the screen, but the volume+ button is a better way to capture the image. It also helps with #4.
Happy Independence day from the eCatholic Team!