Divide the project into major components, then tasks and subtasks. Continue breaking them down until they are each straightforward tasks. They should be of short enough duration that timeframes can be accurately estimated.
This can be done within the Gantt chart table, with a hierarchical list of goals or work divisions, with tasks and sub-tasks listed under them.
It may also be done visually, using a mind map. Many project planners find that it's much easier to think and plan a project in this way. When you use SmartDraw to plan your project, you can use both, because the data you input in your Gantt chart links to a mind map, and vice versa. This allows you to create your plan using either or both of these formats, without having to input information more than once.