I assume you think on the purpose and desired outcomes of your project first. Then you brainstorm, perhaps with a mind-map. Organizing your ideas may take various forms. But at minimum, joy down tasks that you need to get done to achieve the desired outcome. And decide what your very first action will be.
When you complete these steps, Know that you’re already ahead of 80% of other project managers. The rest depends on the size and scope of your project.
If yours is a large scale project, have more lists: of stakeholders, deliverables and milestones, cost items, risks and mitigation strategies, project team and responsibilities etc. Consult conventional project management methods and consider using more elaborate tools while using GTD as a general guide.