How to write a business plan for YOU, why you need to write it out and how to use it. Free template and video walk-through. You don't need a 20 paged plan if you're not getting loans or investments. You just need all your ideas in one cohesive outline.
Business Plan Mind Map - that's exactly the level you should look at this topic in an early stage startup environment. Use it as a brain teaser, be very selective (e.g. don't worry about balance sheets etc.) and don't overstrategize. Stay humble and lean and capture your initial data in the lean or business model canvas - not in a heavy business plan document.