Instructions
Determine the rules of the world you are going to create in your role playing game. While your game can certainly be set on earth, most RPGs are set in different worlds with different rules.
Determine what characters are going to be part of your role playing game. With each character write out a detailed biography of that character, his or her powers (if any), and if possibly draw a picture of that character so gameplayers can get a feel for who they are.
Create goals and achievements for your game. Every game needs to have some sort of goal, right? Determine what you want players to try and accomplish while playing your game, and write out those goals. Goals can be group-oriented, such as "Determine who the murder is," or can be character-driven, such as making a goal for one player to befriend another, or learn a particular piece of information.
Determine how your game will move forward. Will their be a die and gameboard? Will the characters just talk to each other? Set some ground rules, be them time or otherwise-based that keep the game moving.
Put all of your pieces together and have some friends over to give your game a test run. Chances are you'll need to play the game several times to see things you've forgotten and to work out kinks in game play before your game is ready to be shared with the the public.