When you gain access to the shipping container full of fireworks in Karl’s Bay, you will find a terminal inside that you need to interact with. By sabotaging the machine that controls the fireworks, you will be able to kill Ramblin Frank by making him kill himself.
Picking the incorrect option will not really impact you heavily as you can always go back again in the future and select a different option. If you have not been paying enough attention or just forget what the option was here is what you do.
How To Sabotage Ramblin Franks Fireworks
This option may differ depending on the game, I am not sure if it is a randomly generated option. For me, the option was to mess with the flaps.
If you check your quest log, you should get a clue in here somewhere. It will mention that you should disrupt the rain flaps because it is an area of the fireworks that Frank is not well educated in and is likely to overlook any issues with it.
The video below will show you what you need to do as part of this quest and how to mess with the fireworks to ensure Frank is killed.