Caveat: I am unpublished, and I won't begin writing my first book (of a planned series) for another couple weeks. I don't recommend my approach for those who lose interest in something relatively quick. For me, I enjoy the environments and pieces I made. I have a lot of material to cherry pick.
I am very strong with project management and creativity. I know I wanted to create something big and I know I wanted to do it in the fantasy genre.
1. My first task was to survey the landscape. I learned all I could about fantasy characters, beasts, magic systems, and a seemingly endless list of items.
2. I learned all I could about writing novels and the business itself. There was a long period of this for me. I wanted to know what I was getting into.
3. I created pieces: loads of characters; new kinds of beasts and modifications on existing ones; 800+ spells and potions; fun lists (e.g. new food items); and so on.
From the created pieces, I had something I could work with; i.e. things that helped me create things such as setting, plot, subplots, tension/conflict, and so on. For me, it's like the 1980's cartoons, such as G.I. Joe. Each side (good and bad) had numerous characters with specializations, vehicles, and so on. I can cherry pick the most entertaining characters (human and animal) I created and put them into fun situations. I can create stories (plots and subplots) from these.
I left out a bunch of stuff, but now I have books one and two outlined; each with subplots and, I hope, great quality. I have some more odds and ends to wrap up before writing, but the entire process will take, if all goes well, exactly 1.5 years (to the day). Originally, I guesstimated 6 months. Boy was I wrong!
There were a lot of problems I never encountered before, and this is where my project management paid off. Although I didn't know how to tackle certain elements, I had experience in doing relational things. One of those was learning protagonist layers of support and lack of it. I studied the Harry Potter series to work through this. I figured out Rowling had 5 positive support layers and 5 destructive layers. I learned she worked it, so that the most powerful (level 5s) attacked levels 5s of the opposite support structure. For example, Voldemort (detriment level 5) killed Harry's parents (support level 5). At level 4, LeStrange killed off some level 4 supporting roles for Harry. Rowling had a 5:7 ratio (support layers to number of books). I adapted the ratio to my planned number of books and went on to the next challenge I struggled with. This allowed me to know who kills who and when. I learned Rowling's kill ratio off good people dying to bad characters dying. I applied the same ratio to my WIP.
For the amount of work put in, and finding creative ways to solve problems I encountered, I am surprised it only took me 1.5 years (working seven days a week, even holidays) to get to where I am. It's a huge project, but in the process of creating everything I did, I created numerous side projects. I have 30+ side projects from animal and human characters I could never use in my series, but I created during the process. I created story lines for them. One day, I will try screenwriting with these projects.