I just listened to "GTD Fast" and got a slightly different understanding of the natural planning model. Please confirm.
1. Brainstorm / identify the purpose and vision.
2. Brainstorm everything you can about the project, use trigger questions and, ideally, mindmapping. Place #1 at the center of the mindmap.
3. Use #2 to build an outline/set of tasks,etc. Immediate next actions should go straight to your lists.
I also got the feeling that #3 was best focused on sub projects and deliverables, rather than detailing every possible step that you might ever need for each.