When you're writing a business plan in a situation like these, you need to address a few issues the intentional entrepreneur has already pondered. The first is do you really want this idea to become a full_blown business? Certainly it's flattering when you realize there's a market value for something you were doing anyway, but that doesn't always mean you should launch a business. A lot of accidental businesses form around fads or seasonal items, and may not be robust enough to function as year_round, money_making, enterprises.
So why make a business plan? I want to show you a totally different kind of business plan. What if you made a business plan that focused only on what you want for your life? You have dreams about what you would like your lifestyle to be, right? Why not make a business plan that could give you those dreams? What would your business look like if it gave you exactly what you want in life. What kind of salary would your business need to give you? Why not build a business plan around that? Decide how much salary you would need to support your dreams and then build a business plan that would show exactly how your business could give you that. Wouldn't it be better to have your business work for you instead of the other way around?