Flash would not be switched on in video mode anyway. Aperture to smallest? That would reduce the amount of light. I'm guessing you mean set to largest? Not really, fireworks are bright in case you haven't noticed. Setting to f/8 is enough.
The basics are:
Mount body on tripod
Connect remote shutter release
Set to Manual Shooting Mode
Set ISO to 100
Set Aperture to f/16
Set Shutterspeed to bulb
Cover viewfinder with sandbag or VF curtain (optional)
Reposition tripod head to get your framing and focal length right.
Set focusing to manual and focus to infinity ... provide the subject is in focus at infinity, usually they are as nobody will take fireworks close up.
When firework starts, hold remote shutter to open for 4 seconds. See if your shots are overexposed and adjust accordingly. If they are overexposed, hold for 2 seconds. If underexposed, hold longer.
Do not worry if the background or landscape or buildings are underexposed because you can correct them at post if you want. If your fireworks are over-exposed, they are almost impossible to correct. So get the exposure for the fireworks right first.
So these are the basics. They are not hard and fast rules, if you know your basics, you will know what to adjust. Now go practice.