Instructions
Come up with a central game concept. The game concept should include the genre your game will encompass. Choose any genre that appeals to you, from fantasy to science fiction to horror. Include the game world in your concept idea. The game world includes such background information as geography, political systems and populations. Include as much detail in the game world as possible. If your game is nonfictional, research the setting and as many real-world details into your game world as possible.
Create the rules that determine the actions your game characters can perform. Begin with a system of statistics (or "stats") that relate to general ability. The statistics include those things most important in your game world. For example, in a horror game an included statistic might be bravery, denoting how resistant to being frightened your characters may be. Create a skill system where the characters can increase uniqueness by portioning out their abilities among certain skills useful in your universe. A modern game might use driving, marksmanship or computer use as skills. Be careful not to overrun your game with skills, though. With too many skills, the players may become confused as to which to use to perform tasks.
Create a set of rules covering the creation and advancement of characters for your game. A simple rule set allocates a number of points for use during character creation. Players allocate the points among the various stats and skills to gain proficiency. Points used to raise stats should be greater than those used for the more specific skills. Prevent the creation of unbalanced characters by setting limits on point allocation; for example, limiting the number assigned to each skill or stat. Also create a system for advancing the characters as the game is played, so that more difficult activities can be attempted as the characters become more proficient in your game world.
Create a system for handling player conflict. Cover any general area of conflict from violent encounters to haggling for a better price on a motel room. Create a system that's flexible enough to handle any situation that may arise, while still being based in a set of easily understandable and consistently applied rules.
Flesh out the game world, creating a starting point for characters along with a beginning adventure. Guide the characters through the world, adding details to the world as you go along. With each subsequent play, your game will grow.