Things You'll Need
Flow Charting Program
A video game script is one of the most difficult writing projects you can undertake. Every detail of the game has to be described. Every decision the player makes has to be diagrammed, and the results have to be flow charted. This is not something to just dive into. If you want to write a good script, be methodical about it. Plan obsessively.
Come up with a concept. Who is the protagonist? What does he want? What does the world look like? What obstacles does he face?
Decide what kind of game it is. Writing a video game script for a first person shooter, simulation, or real-time strategy game is, in general, much simpler than writing a script for a role playing game or something else fairly interactive. In an interactive game, you have to deal with multiple, branching plots. In most other types of games, you have a linear plot where the player simply has to beat each level to go on to the next.
Write an overview of the story. From beginning to end, describe everything that happens in the game. If there are multiple branching plots, don't feel the need to explore every one in detail at first. The main purpose of this is to get the grand story of the game planned out so that you can go back and flesh out the structure.
Write backgrounds for the characters, worlds, and any significant objects in the game. Include personality details, physical descriptions, and back stories for everyone and everything important to the main story of the world.
Write a flowchart. This documents every major decision the player faces, and the results of the decisions he makes. It should consist of text boxes with arrows to indicate what happens when an action is performed. If there are multiple decisions the player faces, "yes" and "no," "left" and "right," etc. can be used to indicate where the chart branches.
Write out interactions with non-player characters (NPCs). You will have to write out flowcharts with the NPCs showing how they respond to what you say, do, or give to them. You should also flowchart any puzzles, mazes, or quests the player faces.
Choose a formatting style for your script. The style you use does not matter that much, as long as you are consistent. The sample script linked to below has good formatting which you could use.
Write the actual script. Include dialogue, camera angles, what the player sees, what decisions he has to make, and what the programmers need to know. This will include any important rooms, objects, or NPCs that will be encountered in each episode. You can also include notes on music played, sound effects, or environmental effects. For example, if you want clouds to role in at a certain point, making it hard for the player to see, include that detail.
Don't feel the need to write back stories for trivial objects or characters. A thug who the player has to fight can be pretty two dimensional, and a knapsack can just be a normal knapsack.