Design Document is not crucial for the success of your game. But it can be crucial for you.
The main reasons for creating a design document are:
- To present the game for others
- For qa, as everything that differs in the game from what's in the document, is a bug
- For you to clarify your thoughts
The last one is very important for you, as it is very hard to get other to read your document. Even you get someone to read and use your document, it will be out of date pretty soon. Therefore you need to be presenting the game to others all the time.
When you create your document, it's not about text. It should only or mainly contain:
It should only contain sentences that require action from someone, you, artists or programmers. This will make it easy and fast to read and clear. Nothing will be hid behind text.
Here's an overlay sample of a game design document.
But, the main concern is not to create the design document, but to keep your team informed about the game. It is more important to communicate the game to the team, than to the document.
Anyways, you may find a design document template from Files.