Do you have a story burning inside you, waiting to be made into a novel? Everyone's journey from idea to novel is going to be different, but I'll detail here what has worked for me. First I should note that all of this is flexible. The actual journey from idea to novel takes many twists and turns and changes constantly.
Start out with a sentence. A single sentence. This sentence should encapsulate the entire book. For instance, this is the sentence I used for my first book (spoiler alert): A tenacious engineer instigates an interstellar war after uncovering government secrets.
After you've got your sentence, expand it into a paragraph with more detail in it. If you think your story neatly separates into a few parts, give each part a couple sentences in your paragraph. At this point, I pause from my outlining and write character sheets. These are details about my characters that range from one sentence to a paragraph depending on how complicated/important the character is to the story. Each character sheet can contain as much or as little detail as you want. Whatever you think is important: what they look like, what their history is, what their temperament is, what their strengths and weaknesses are, their aspirations, fears, etc. I suggest making a character sheet for as many characters as you can think of that will be in the story, both main and auxiliary. This will help you keep your descriptions and personalities consistent after the writing starts.
After I've made the character sheets, I'm pretty much sick and tired of planning and itching to write something for reals. Then, I write the first chapter. Now I should say that there is much more planning to come, but the first chapter tends to be fairly amorphous without much plot progression, so it's a nice testing ground for your story and to introduce your characters.
Other people would probably disagree with me, but I advocate writing the first chapter before doing substantial plot planning. I find it really helps tweak your characters' personalities and dialog in such a way that planning just can't. You may have planned out an entire book, but then when you start actually writing it, you figure out that the events, conversations, or personality traits planned just don't feel authentic. Writing your first chapter also serves to get you excited and pumped and helps get you through the next part: the dreaded chapter guide.
After I get a feel for what my main characters are or are not capable of (and what I'm comfortable writing), then I go back and make the chapter guide. Again, I start out with one sentence summarizing each chapter, then go back and expand each sentence to a short paragraph. Again, have as much or as little detail as you'd like in there. After you're chapter guide, you have a road map!
Then you just write the darn thing :) Some tips on this: change is expected. When I'm writing a scene, I know the general direction things are going to go, but I don't necessarily know how I'm going to get there. This is going to sound strange, but I let the characters show me how the scene will play out. The scene will feel much more organic if you get in the mindset of your characters and record their reactions to things as you go along, rather than imposing actions and words on them ahead of time. I will frequently have something in my chapter guide that seems great, but when I'm writing it, it just doesn't feel right for whatever reason. So be flexible, and adjust your story as needed as you go.
Last suggestion: don't go in with the mindset that you are going to write the whole thing at once. The quickest way to intimidate yourself out of writing your first book is to think about how much you have left to write before your first draft is finished. I take it one scene at a time. Each scene you write gets you closer to finishing your first draft. With the rest of my busy life in the balance, I aim to write about 2,000 words a week currently, which is very manageable. Before I was working 50+ hours per week (and before I became a dad), I was easily writing about 10,000 words per week. So when life gets busy, it's okay to alter you writing habits. That being said, I wouldn't take a break from it entirely. The only way to keep your skills up is to keep writing :)
In summary: plot it out, be flexible, let your characters write the scenes for you, and set reasonable goals for yourself. And above all, have fun! Unless writing is your profession, you should be enjoying it more than stressing about it. If you have any questions about my process, or comments, feel free to leave them below or contact me!