Instructions
Choose the object of the game. The simpler the objective, the better.
Make the rules. The rules create the challenge of the game. A children's game may have a simple set of rules, such as get tagged and you're out. Adult games will need more complexity to hold interest, such as requiring a team to solve a piece of trivia before moving on in the game.
Write down instructions for your game. This will help you filter through inconsistencies in the rules and simplify the explanation before you present it to the teams.
Choose prizes or outcomes. An outcome for a children's cooperative game, for example, might be all teams cooperating to solve the last problem; this also teaches children the importance of teamwork.