Instructions
Create your setting. Before you ever begin to write rules for an original RPG, you want to have some idea in mind of the RPG's genre. Genre impacts rule choices significantly. You need to know early on whether your original RPG setting will be fantasy, science fiction, horror, anime or modern/realistic.
Develop your core mechanics. Core mechanics include combat resolution, social interaction, movement, attributes, skills and abilities. Pay special attention to the mathematics involved in your core mechanics.
Design character templates. Examples of character templates include class and race in "Dungeons and Dragons" or clan and covenant in "Vampire: The Requiem."
Decide on how advancement works in your original RPG. Considerations to think about are a level-based advancement system, characters that can gain new skills or abilities, or advancement that relies solely on the acquisition of items.
Develop enemies for characters in your original RPG. Examples of enemies include monsters, animals or other people like the characters, but with evil intentions. Design enemies that are roughly equal in power to the characters.
Play test your rules thoroughly before you publish them. If possible, play test the rules for your original RPG with many different people. Ask others to use your original RPG with their own gaming groups.