Do some research:
First and foremost thing you need to do is to understand the reality of that world. Do some genuine research instead of being content with movie cliches. Though, you do not need to do conduct proper field research but you should at least read newspapers and journals to understand how gangsters live their life and how they make money. This research will, really help you to bring your story close to the reality.
Examine the basic structure of story:
You will find many novels with the theme of the underworld mafia. Try to learn the basic structure of a gangster story. You will notice that most of the gangsters started as an honest person but then injustice or any personal tragedy transformed their personality. Mostly greed and revenge are the driving forces in these types of stories.
Pick your locale:
Earlier, underworld dons were based only in Italy or Ireland but now this business has expanded to Russia, Asia, Mexico and Columbia. They all have their own style of living and doing business. It is very important to pick a locale and then portray it with perfection. For that, you will have to focus a particular mafia to bring uniqueness and perfection in your screenplay.
The next thing you will do is to develop characters. Obviously, you need a protagonist or protagonists to run your story but you also need some secondary characters to ensure the smooth development of the story. You do not need to present the main character as an extraordinary person but make sure the protagonist is strong enough to head a mafia.
Write down the plot:
After finalising the character, developing a story should not bother you as your characters will lead you in this. Write down the main incidents, turns and twists of your story.
Now, when you are done with characters and story, you should start writing it with dialogues. Complete a rough draft and then review to make amends.