Start to develop the structure and don't worry about minutia for now. This will help you develop transitions from chapter to chapter. It will also show where you are weak in your some thoughts and where you need to fill in the blanks. Connect the dots from beginning to end.

Don't worry about putting down too much of your thoughts. Eventually, you will learn to edit, pull out the extraneous information later, to possibly use in another way, such as another chapter or put in another work.