Set the exposure to M and “bulb” use a remote to open the shutter when the sky is clear and wait for a few fireworks to go off and then close the shutter. Set the camera to f8-11 at ISO 100-200 and focus on infinity. I use an infrared remote. You could also use a cable release.
Another way to do this is the “hat” method. Set your exposure for 30 seconds or so and cover your lens with a hat. Move the hat just before the firework goes off and then cover the lens with the hat. Do it again and again. You will get a number of fireworks on one exposure.
A sturdy tripod is a must.