Research real-life gangsters from various eras. This helps you understand where they came from, how they operated and what kind of personality they required to thrive. Pay particular attention to the way organized crime works and how it differs from more mundane types of crime.
Look at gangster movies featuring famous characters, such as "The Godfather," "Goodfellas" and "The Public Enemy." These can provide excellent inspiration.
Determine the ethnicity and era for your gangster. Most classic gangster stories take place in the 1920s, but organized crime can be found in any era, and modern gangsters comport themselves much differently than earlier bootleggers. Similarly, ethnic background plays a large role in the gangster's identity. Many fictional mobsters are first- or second generation immigrants, with strong cultural ties to their homelands.
Develop a history and basic personality for your gangster. Where did he come from and how did he enter into a life of crime? Was he seduced by the promise of power and riches? Or was he born into it and now wants a way out? Note specific incidents in his life and think about how that might affect the kind of person he is. In particular, you should discuss the types of crime he is involved in and what role he plays in those operations.
Come up with some rivals or enemies for your gangster. Every mobster has enemies, whether it's a crusading local DA or a ruthless criminal rival. They will determine how your character reacts to danger and provide obstacles to overcome (or succumb to).
Flesh out your gangster's personality based on his past history. Is he a raging hothead like Sonny Corleone? Or a ice-cold schemer like Sonny's brother Michael? Does he have a moral compass? If so, where and how does it come out?
Describe your gangster's physical appearance: size, physical attributes, the kind of clothing he wears and any tics or twitches he may demonstrate.
Think about how your character might die. Things rarely end well for gangsters--it's the tragedy of their story--and the end they make for themselves has as much to do with who they are as their life and crimes.
Write down all of the details you have come up with in order to refer back to them. If you have artistic inclinations, you can draw a picture of your gangster as well.