Minimum equipment required:
Camera - with a T (Time) or B (Bulb) setting so that you can hold the shutter open.

Tripod - Strong and sturdy.

A black card - I made one from a piece of foam board, about the size of a 3x5 file card. Spray the card with flat black paint.

Camera settings:
Shutter - T or B
Focus - infinity
ISO - 100
White balance - incandescent

Set your camera on the tripod and aim it toward the area where the fireworks bursts are expected. Lock all the movements to eliminate any camera movement.

You can usually see the fireworks as they rise in the air before the burst goes off.

Cover the lens with the black card and open the shutter. As you see a shot rise, remove the card and leave it off until the burst starts to die off. Replace the card but do not close the shutter. Do this two or three times, then close the shutter. This is how you get multiple exposures in one frame.

Repeat this process throughout the fireworks show. Don't try to get too many bursts in each frame. Often less is better.