The business plan is a necessary but not sufficient condition for finding outside investors. The plan describes the business and what it might become, and that`s all. A beautifully written, edited, and formatted business plan will not make a less investible business more investible.
First, you want your business plan to be read. No one is going to read a 100-page or even 40-page business plan. Sure, you may need supporting documentation for specific sections but you can include those elements in your Appendix.
Highlight the key aspects of your financial plan, ideally with a chart that shows your planned sales, expenses, and profitability. If your business model (i.e., how you make money) needs additional explanation, this is where you would do it.
There are tons of good outlines available, plus books, blogs. Down below I have some specific resources related to my work; but not now. There is no single best outline to use, but investors will want to know about the market, potential growth, competition, differentiation (or secret sauce) strategy, tactics, key milestones, important assumptions, the management team, and financial projections including use of funds, projected sales, income, balance, and cash flow. Use your common sense to put first things first and organize it all well.
Defining the problem you are solving for your customers is by far the most critical element of your business plan and crucial for your business success. If you can`t pinpoint a problem that your potential customers have, then you might not have a viable business concept.
Now, you may not have every milestone or even specific steps in mind to reach your goals before starting. But that`s the beauty of working through your business plan. It will help you define metrics of success, flesh out your goals and further develop elements of your business to meet specific objectives. You just need a vision or even aspirational goals to start with to better hone in on what`s important.