Oh dear, it's so simple to write a Business Plan. Simply start jotting down the points about the way you wish to do your business. It's your Business. It will be your Business Plan.
Then download a template and start plotting your points under respective or nearest captions in the template.
You will find most of the captions covered. If anything is still left out, you may seek the support of a professional business plan writer.
Best of luck to every Entrepreneur.