Some writers plan thoroughly, creating detailed outlines and notes before committing any words to a manuscript. Others write completely by the seat of their pants. Many exist along the spectrum in between these extremes.
I'm closer to the pants method of writing. I create very high-level outlines, something closer to a list of milestones, usually after I've written a good bit of the piece. I make the occasional note when I have an idea to be used later, or when I'm working on world-building notions. For The Emris I needed a timeline to track the multiple threads of action and tie them together properly.
I don't believe that there is any right or wrong way to go about it. What works for one individual won't work for another. If you can't stand the thought of outlining, don't. If you can't stand the thought of writing a single word without a thorough plan, don't. Find your place along the spectrum and reside there with confidence.
That's my take on it, for what it's worth from someone who has only recently ventured into the world of writing.