The business plan is your key document for ongoing planning. The business plan establishes your financing needs, documents your research into funding possibilities, and reflects advantages and disadvantages of various legal structures, to help guide you to the right decision. Starting your own business is like building a house: you need a precise plan with carefully considered and realistic calculations.
Write your business plan yourself. As a self-employed person you have full entrepreneurial responsibility for the estimates, plans, and decisions described there.