Don't get confused by the needs for a business plan. Excuse me, but I beg to differ. You'll need a business plan if you need a bank loan or some other kind of money. Apart from that, please don't bother. Business plans are worthless in my opinion. Here's why:
Let me give you an example:
"We'll find 250,000 buyers of our game after 11 months of development with 6 people!"
If you think about it, every single number mentioned means a lot. 250,000 buyers might just be enough to make some money, but maybe you'll only find 125,000 buyers. But the ugly part is this: How would you possibly know? Look into a crystal ball of knowledge? If you got one, I'll buy it off you for any price.
11 Months of development. Yes, that's the plan. If you realize you need more time, then your business plan instantly has become totally worthless. You'll still have to cover costs for additional months, you maybe need to find new funding. Nothing, really nothing in your business plan will work out if you need more development time. This does not automatically mean your business has failed, but your business plan has become outdated. Thus it became worthless.
Development with 6 people. Yes, but what happens if your unpaid labourers leave you? At least half of them possibly leave you after 4 weeks because they get hired somewhere else - where they get paid. This still does not necessarily mean total failure: You might find a way to get funding somehow and yet you might even finish your game with only 3 or 4 people. No business failure, but your business plan won't be worth a dime anymore. Because it was based on development with six people.
It is completely natural for any kind of business to change over time. Maybe change very often. You might change the genre of your game you're developing, you might change parts of the team or the game engine being used. You play the alpha with some testers and suddenly realize: You'll need to add multiplayer support or noone on earth will ever buy your game. Being able to change and to adapt is vital to your business. A business plan on the other hand is very static. It won't help you creating your business unless you need a bank loan. But what it really won't do is help you maintain and change your business. Just imagine keeping your business plan up to date - rewriting it every single week - altering your assumptions to the future of your business, adding new assumptions? That is not viable.
What else could you do? Google for stuff like "business model canvas" and various other stuff related to starting your business. Read books, take courses, watch webinars. Don't fall prey to dubious instructors, though. There are many out there.
Or, simply put: become an expert at building your own business, become an entrepreneur. You don't need to learn everything business related. Example: you don't need to learn accounting. Rather, hire a professional for that. It will save you time and money. But you do need to learn how to be an entrepreneur.
I do respect other opinions, but these are my two cents.