Describe the manufacturing or operating process and associated costs. Contain believable finical projections, with key data explained and documented.
But do not be too paranoid, make sure you are showing it to people you trust, whose feedback you welcome and can be genuinely useful in guiding how the plan takes shape. Very often when working as individuals we get very close to certain details and miss out a big thing that has slipped your mind.
I say that because of course you need to be careful with commercially sensitive ideas, as you do not want to pass your plan on to someone in the pub who then starts your idea before you across the road.
For any organization, various functional plans-marketing, financial, human resources, production-have to be laid down, even though all the functional plans serve different purposes and all theses plans provide guidance and structure to the successful running of the business.