Different writers have different approaches. Some plot everything out in detail before they begin. Others begin with an idea or a character or even just a picture and start writing to explore from there.
Even for the pre-planners, though, writing is an act of discovery. You are going to learn things about your subject matter, about your characters (in fiction), and about your own ideas as you write. This will force changes to the plan.
I think it is fair to say that writing is recording that which has been imagined, so I think the question becomes: when and by what process does one's imagination form a picture or an idea that is clear enough to write down. For some, a plan may be necessary to reach that point. For others, a plan may lead them away from it.
Personally, I find the plan comes somewhere in the middle of the process, as a means to understand how the threads of things imagined can be brought together into a cohesive whole.
You need to find the thing that gets you to that point at which you have something clearly in your head to be recorded, and then start writing it down.