Thus, plans will always be least accurate at the beginning of a project, and they should get progressively more accurate as the work progresses. This suggests three things. First, you must plan, even if you don't know very much about the job. Second, you should recognize that the initial plans will be the least accurate. And third, you need to continually remake the plans as you learn more about the work.
Planning is much like designing a product.
It is a good idea to start with an overall architecture or process and then to lay out the entire structure. Until you have an overall framework, a detailed plan could address the wrong tasks or focus too much effort in the wrong places. For example, I was assigned to a project some years ago. There had been many small schedule changes, but everybody thought the project was on schedule. Nobody had ever produced an overall plan.