The Pepperdine Libraries have many books on writing business plans. Type "business plan*" into the search box (include the quotation marks so the term will be searched ast a phrase) for books about business plans.
Researching private companies is more challenging than publicly-traded companies since they are not required to report information to the Securities & Exchange Commission (SEC). In most cases you will not be able to obtain financials statements such as income statements, balance sheets and cash flow statements for private companies.
One approach that students have used in the past when researching privately-held companies is to locate a similar public company and use their financials as an example or proxy.