The purpose of a creating a games design document is to have an ordered and descriptive layout of my design choices in a way that it provides clarity and understanding to those I need to convince to develop my game idea. It needs to provide enough information that it could be taken away and the game be developed.
Where to begin:
Pitch - The pitch is a brief one or two sentence description of a games essential experience, foundation and key features to summarise the game.
Overview - A content summary. This includes the story, core mechanics, the gameplay experience (descriptions, prototypes, storyboard), UI design, target audience, platform and controller mapping.
Design intentions - Document how the game will looks.
Who are the characters?
What are the environments?
Are there any assets or customisations?
Main and side quests.
Major/Minor Focus content - your specialisms
Demonstrate chosen foci as they would exist in the game. Provide opportunities to showcase your specialisms and intentions.
Other things to consider:
Asset Showcasing - animations, concepts and environment mock ups.
Demonstrative prototypes.
Evidence showing you have taken other factors of game design into your decision making.
Technical limitations.
Narrative impacts.
Mechanics.
Music.
Ideation and research.