Sit down and type until it’s done. That’s the short answer.
If the book is part of a series, I definitely do a lot more outlining and preplanning. Picking names is the first challenge. For virtually all of my books, I do a brainstorming exercise where I list all the reasons the hero and heroine resist each other, and all of the reasons they are attracted to each other. This starts to form the emotional arc of the romance. I often write a few paragraphs of back story about each character. About 70% of the time, I write a synopsis, then change things as I go along, so what’s the point in that? Not all of my publishers ask to see early chapters. Lately, I’ve been submitting full manuscripts. My editors offer feedback and I’ve been lucky in that I’ve never had a truly awful experience with revisions.