I’m pretty sure no one can say the “Right” or “Wrong” ways to write, much as no one can say the right and wrong way to draw or paint or sculpt or any other form of art. Do what feels the best to you. Writing a full draft will probably help you stay focused, but at the same time would you want to squash a sudden idea to improve a previous section? I say just start, and see how it goes. I’m also not really a writer, though, so take my advice with a grain of salt.