Following the example docs provided is a good way of doing it.
If you find that doesnt work for you, you can come up with your own method of doing it.
The important thing is actually doing it. It helps you get your ideas down on paper (screen). It helps you. Gives you a kind of roadmap so you know where you’re going, what you’ve already done, what you want to do, and makes it easier to put down new ideas so you dont forget about them.
I have my own system. Its less formal then the GDD, but works better for me.
I have a 100 page composition notebook I use.
The very first page is my index, where I write what is on each page.
Then when its open, I use the left page for sketches and flow charts when required and
use the right page for the actual idea.
One idea/function/problem per page. Make sure you put a title or brief description on the top of the page, then if your notebook isnt already numbered, put a page number in the top corner, then add that page to the index at the front.
If I ever find it necessary to share this with others, or want to clean it up, I would then type each page in, and scan the sketches and diagrams.
I use OneNote by microsoft for this, its great for organizing things, then I can save it as a PDF to share/distribute.
If you use a 3ring binder, its easier to reorganize things, but I’m left handed, so 3 ring and spiral bound notebooks were the bane of my school life.