A mind-map evolves naturally from a story core and, in essence, embodies and extends the story core by providing new information about the plot and more in-depth consideration of elements of the story's core: problem, transformation, and resolution. If you elect to use mind-mapping to start your process, then you will find that it's easy to do and costs almost nothing to create except time, collaboration & creativity. 

What do mind-maps look like? They can take many forms. Take a look at a variety of them, and decide which tool works best for you. As with all things in storytelling, there are guidelines but no formulas.