When you are writing a book having a few documents on your computer will help greatly in collecting your thoughts. I think the best place to start is getting your plot laid out.
Even when publishing a book, especially when writing a proposal, it is expected that you have a plot outlined before you begin your writing phase of your manuscript. So what you should do is get together a word document and write a basic summary of the direction of the book. This will be a good guide to start with but try not to follow it religiously, otherwise you'll miss out on some good organic writing moments. Usually a plot outline will consist of a paragraph of information laying out the primary "beats" of that chapter. If you've ever read comic books, beats are essentially scenes that your book will consist of. No matter how James Joycey your book is there still are beats that compose the primary arc of your narrative. Construct a cohesive sequence of beats for your plot outline, maybe a sentence each and then repeat for each chapter until you have it all laid out.
Another essential step is collecting whatever scraps of detail you have and putting them down on paper/word document for storage and recovery. This process is where you get detail intensive. Where I start is composing a mission statement for the book, what I desire to accomplish most in the writing process. Afterwards I build a map of where the book will take place. This is incredibly important. A map is what establishes your world, making it interactive and relatable. Without such a map you will never really ground your character. The opposite is also true. If you want your world to be ethereal and confusing, it's best not to create a map, otherwise it'll seem artificial and too structured to be real.
Characters are the next step.
Write short biographies for each character, including their names and occupations. Also include their descriptions and certain perks that make them who they are. Below is an example to get you brainstorming what this would look like in your document.
Ray Wilks - 35 year old auto mechanic, honest and forward about his lack of discipline. Has a 20 year old girlfriend with perky breasts who thinks that Ray will someday own his own shop. He is fit but lacks altheticism, smokes and owns a yellow, rusted Dodge Charger. His father used to beat him growing up; therefore, Ray goes to church, which he believes will make him good and moral. He is not intelligent, but knows cars with a surprising level of expertise. His favorite hat is orange with white racing stripes which wrap over the top of his head.
The description is not supposed to have any sort of internal conflict or character building elements. This will come later.
For the rest of the document I put in relevant information concerning the world. What are its politics like? What are the "issues" the world battles with? Are there opposing forces that try to shape the world contrary to its current form? Ask these questions and document them. This will help a great deal when writing.
Again this not an exhaustive overview but my hope is that you will use these brief examples to better articulate your writing. Until Thursday, I hope your writing continues well!