When I say 'Game Design Document' to most indie or hobbyist devs they immediately recoil in horror. "GDDs are something Big Games have!" they say, thinking their game is not "good" enough to warrant a design document. Or they scoff "GDDs are for marketing executives who don't understand games!" and go about making their game without one, thinking they'll never be such a sell-out as to need marketing executives.
As an indie game dev, I'll let you in on a secret.
They're both wrong.
Every game, big or small, governed by executives or by one bedroom programmer, needs and deserves a Game Design Document. I'll tell you a little bit about why.
The GDD forces you to think about your game, and not just in hand-wavey "Big Picture" terms or in small-detail technical nitty-gritty terms, but as a whole picture. It makes you look at each element in turn and go "Hm, where does this bit fit in my vision?"
So I'm going to help you create a GDD for your game.
First step is to create a document with seven pages. I've lost some of you already ("I don't have time for seven pages!"). Title each of those seven pages as follows:
-What is [insert name of your game here]?
-Characters and Settings
-Items and Weapons
-Breakdown of Components
-Breakdown of Assets
-Suggested Project Timeline
Now you've titled those seven pages, you're halfway there. No, really. Knowing what to put in the GDD is half the battle, and I've just told you. So lets look at each of those in more detail.
WHAT IS [MY GAME]?
Excuse my caps but the underline function isn't working.
This is where you talk about your game on a high-level, hand-wavey platform. At the very least you should detail your expectations (what do you expect the finish game to be like? What do you think players will take away from the play experience?), your mechanics (how does your game play?), your aesthetics (how does your game feel?), and your game objectives (what is it the player is trying to achieve? What will they receive from doing so?)
If you don't quite understand mechanics and aesthetics, or the difference between them, don't worry. I'm going to write another tutorial on that.
CHARACTERS AND SETTINGS
Here's where you should talk, briefly (a paragraph is about right), about each of your main characters. By 'main characters' I mean playable characters, or characters you can interact with. Monsters come later, in another document ("Oh no! Another document!") which I'll detail in another tutorial.
You should describe in a paragraph each of your main settings (key areas to your game, use your discretion to decide what's a 'key area'), and provide sentence-length descriptions of any supplementary settings.
ITEMS AND WEAPONS
You should not only list all of the items/weapons in your game, but where they're found and at what stage of the game. If you haven't thought about your game in any real depth, now is the time to start doing so. It's also a good idea to attach maps, concept art, or whatever you can at this stage to help flesh out the game world. This is where we really start to think about the minute-by-minute play of the game, so think hard.
BREAKDOWN OF COMPONENTS
Here you're going to talk about things that are needed to make your game work, but don't constitute part of the gameplay or story. Examples include the engine you're using, saving/loading mechanisms, the heads-up display, and any key mechanics which are unique to your game.
BREAKDOWN OF ASSETS
This is where you really detail the aesthetics of your game (if you don't understand aesthetics, don't worry, this will make it a little bit clearer). Aesthetics are just as important as mechanics when it comes to designing a game, although most beginner game devs ignore them. You'll want to detail your art style, text, and sound. Anything that gives the game 'feel' rather than 'substance'.
Here's where it's time to start drawing, and to start thinking about the blow-by-blow account of your game. This should be detailed enough that anyone reading it will know how your game will play out, but keep it a little bit abstract to save space and time. If you find yourself writing "start > explore until end", you know you need to think more about how your game will play out.
SUGGESTED PROJECT TIMELINE
This is the only part of the GDD which is optional. If you're a hobbyist and you're making your game for yourself or friends, you don't need to worry too much about a project timeline. If you're serious about your game getting released, this is a must-have. Detail it on a month-by-month level, or, if really necessary, by every two weeks. It should include everything through blue-sky ideas to release.
SOME HINTS AND TIPS
Your GDD is a living document! Once you've finished it don't go "welp, that's that", print it off and keep it in a drawer never to be looked at. Give your document a version, and keep updating it as your goals and the game changes.
Share your GDD! It's great to receive feedback, the detail level shouldn't enable anyone to directly steal your ideas, and it'll help you learn and improve (or discover things you missed).
Have a look at real GDDs, but don't take them as gospel.
Change it up! Is there something missing from this plan which is really vital to your game? Add it in! Don't be afraid to change the document to meet your needs.