To make an effective plan, there is a famous rule called 4P's. Here, P's stand for Price, Product, Promotion and Placement. In this another P is added called People. Today while doing digital marketing or the traditional marketing one should understand people or the customers you are dealing with. It's important to figure out what is significant for the business, and later make a plan accordingly.
Therefore, a marketing plan, according to Kotler, is a "written document, which includes the objectives, strategies and action plans regarding the elements of the marketing mix which facilitate and enable compliance with the strategy dictated at the corporate level".