A marketing plan is a process, not a one time event. It starts by analyzing the market, doing competitve research and research with potential, lost and current customers where applicable. You may want to test your positioning, messaging, pricing, packaging, etc. The plan will also show your budget and the proposed uses of those funds via online and offline advertising, promotion, public relations, etc. Plans should be reviewed for relevancy on a regular basis, many companies have a formal process they go through annually. Hope that is helpful, good luck.