There's an age-old debate on how necessary that plan is. Entrepreneurs can spend weeks to months researching, manipulating projections, solving for gaps in the info... to be too busy to even pick it up once it's go-time.
Further, many of my mentors (who had started businesses many times over) suggested to skip the plan and get right to testing the market and seeing which of the ideas (natural-born entrepreneurs always have a handful of ideas) had the most legs. In a lot of their minds, the plans only got them so far, but the real work came from just doing. And I would 1000% agree.
What's the point, then?
The point is a business is more than a concept. It's a well-timed, supremely prepared for (both in paper and in emotion), and like it or not, a notion full of risk. Do you want to waste your savings and 3 years of good working years to find out what three different projection scenarios would have largely predicted for you? Probably not, because believe it or not, your mega belief in your concept will only take you so far down the road. At some point the numbers will have to start working for you and if they don't you'll soon be working for someone else again.
Is there an alternative to weeks or months of planning?
Sort of. The canvas plan fits on one page, covers the critical points and forces you to think through some important nuances like your customer's buying behaviors and the competitive land scape.
Alternatively, the traditional business plan (as well as the strategic plan that answers anything investors or bankers don't really need to see... like risk mitigation plans or training programs important to your business but not to it's launch-ability) can be anywhere from 3 pages to 30 pages and really thinks through the main questions a budding business owner could think through...especially for their first business baby.
SBA.gov and various online sites will help you draft your business plan. Me or my team over at LyngoSolutions can help you clean it up, really think through your projections, and test the business model before you take it to brokers or investors to trust your preparedness.
Enjoy this journey, even the tedious parts. You're creating something big and you can practice standing back and admiring your plan and all the hard work that went into it.