I would just write down your basic plot in a timeline sorta thing (if that makes sense?) and always keep your ending in mind when adding things in between the significant events on your timeline. i would write down your characters like their physical appearances, age, personality, etc first so that you can better develop them as the story goes on. also think about what you would like to read and you can base some of your style off of that as well. sorry it’s not a great explanation. good luck!!
