I assume you are speaking of a first draft. Each writer must find what works best for them. For me, writing a one or two sentence description of the scenes I want in the book comes first. Then I break the scenes down into bite sized pieces that can be written in a couple thousand words or less. Basically this is an outline. Others writers do the same thing when they set up their outlines. I prefer scenes because that's how I see my stories most often.
Once the 'outline' is set, i strongly advocate not writing the book linearly. In other words, do not begin with chapter one followed by chapter two, followed by chapter three, etc. My feeling is that a writer will get too predictable and is not as willing to go off the outline as much. The outline is a guide, not a written in stone path.
Write the book organically. Pick the chapter or scene you feel most compelled to write at that moment. Hop around. This gives you many varied looks at your book and often allows you to go directions you may never have thought of otherwise.
The MOST IMPORTANT thing to do is kill the critic. Do not allow yourself to rewrite, edit or do any detail work until you get the first draft done. If you write a chapter in your first draft today, the only thing you are responsible for is updating your outline to reflect any twists or wrinkles you may have added. To feel like you must have a tidy, complete chapter before you move on will stymy creativity and bog down the process of writing.
That's how it works for me. I currently have five books in print, two unpublished books and three books in progress. Just remember, you must choose that 'system' that works best for you. One of the beauties of writing is the 'individual' nature of the craft. Each author tends to have his/her own quirks. Most writers I come across who struggle are usually attempting to make each chapter perfect before they move on. This is far too rigid for most creative people.