Story and setting. In the introduction I wrote that you might not even have a story idea yet, or you might have several and you’re not sure which to stick with. There are lots of interesting ways to generate premises. One easy way is to ask yourself what book you would be most excited to read, or to make a list of your favorite things about your favorite novels, and mix and match. You could also review old journal entries or notes and see if you can combine them into something that captures your interest. Ask “what if?” What stirs your emotions? At this stage, don’t worry about creativity or originality. What you want is something that gets you excited to keep coming back day after day.
Action Plan: Come up with a one-sentence “logline” that includes one or more your favorite premise ingredients: unique setting details (in a magical desert kingdom recently invaded by foreigners), interesting elements of the main character (a god-touched princess on the run), a major plot twist or dilemma (discovers she’s been helping to destroy the world, not save it), or the gist of the ending (and thwarts the eldritch plot and resumes her responsibilities to her people as ruler). You can also use the basic template for back-of-book copy to get an idea of different elements to include.
You don’t have to adhere to logline “rules” at this point. Just identify the elements of the premise that get you most excited to write. Feel free to include extra premise elements as a series of bullet points after your logline.
Additional reading: “5 Shortcuts to Getting an Idea for a Novel” from the Better Novel Project, “5 Signs Your Idea Would Make A Good Novel” from Pretentious Title, “Developing Your Idea” from Butterflies of the Imagination, “Where do you get your ideas?” from Neil Gaiman’s blog