One sheet of paper

You do not need a forty-page business plan. You need one sheet of paper, folded into quarters. In the first square, write what you will sell, in a sentence a nine-year-old would understand. In the second, write who will buy it, as a real person, not a market segment. In the third, write how they will hear about you. In the fourth, write what it will cost them and what it will cost you.

Stick it on the fridge. Look at it every morning while the kettle boils. Change it as often as you need to. Mine had coffee rings on it within a week and was completely rewritten within a month, which is exactly what a plan is for.