Instructions
Create a world that your game will take place in. Be sure to create plenty of forests, mountains and towns where adventures can take place. You'll also want to design some caverns and dungeons where the real action will take place.
Design a system of character classes from which your players can create their player characters from. You'll need to include their abilities, including hit points, special skills and whatever weapons or magical spells they can wield.
Create obstacles and encounters for the characters to deal with. This includes monsters or other non-player characters, traps and treasures. Some of these will be placed at fixed locations while others should occur at random.
Write a storyline for the game with a mission for the players to complete (for example, rescuing a princess, finding pirate treasure or infiltrating an evil army). For every game you play, create a different story with new challenges.