How do you write your first book?

You have two ways to proceed. If you are a “pantser,” or “seat of the pants” writer, you just write. The advantage of this is that your writing becomes a voyage of discovery for you, and that might inspire you. The disadvantage is that you don’t know where you are going, and are very likely to run into multiple dead ends and have to go back to an earlier spot and start over.

If you are an outliner, you start with your basic plot and work out everything that happens in advance. I like to start with a couple of lines of premise and main character, expand that to a paragraph, expand the paragraph to a page, expand that to ten pages, then expand that to 40. Figuring 10 pages of prose for every page of the outline, that gets me a 400 page book, or about 120,000 words.