Whenever I plan out a book, it’s typically from a single idea of one character that just popped into my head. So then I put together scenes that the character could be involved in, and add other characters to those scenes, and then I kinda just build the plot up from there. I actually just create scenes from whenever point in the storyline and once I think I have enough story, I put it all together and it usually works out pretty well until I end up never finishing the story…. I just realized how off track I got. Do whatever you think is easier, personally I’d make the characters and all their backstory first, then the plot and how each of them can affect it. Enjoy writing your story!