Writing a business plan is both easy and fun. For a very basic business plan, open a word document and create six sections corresponding to the numbers below.
1) Determine your value proposition.
2) Describe your company.
3) Analyze the products/services you would offer and their place in the market.
4) Determine who will be in your organization and what they will do.
5) Determine what you need to get started (materials, finances, personnel).
6) Create some base line financial projections based on market research.
The Small Business Association has two great templates at: https://www.sba.gov/business-guide/plan-your-business/write-your-business-plan
Score also has a great template: https://www.score.org/resource/business-plan-template-startup-business
Business Model Generation is a great book on the subject: