I get a setting(usually in the past).
I make some stipulations like “I want this one to be shounen action story.” Or “I want this one to be a funny romance.” Or “I want this one to have a Yandere.”
Then I let my mind go and play with it.
Sometimes I’ll make the characters for the story and wind up putting them in a completely different setting or not sticking to any of the stipulations I set in the first place.
It’s not really about sticking to the rules that I set for myself when I make my ideas; It’s about having rules that stimulate my mind and steer me in some kind of direction
But most of all you just got to play with it and work it out and refine it
Edit(I don’t think I really answered your question from what I said hehe): Just look for something that you didn’t feel was done well in a movie or novel or any story. Take that thing that wasn’t done well and do it better. That’s kinda how I come up with ideas sometimes. Seeing how others failed, and fixing it up how I like it>
Like the “strong independent woman” trope:/ I never liked how strong women have to be super bro-ish and tomboy-y, I also hate how movies kinda try to force us to like these characters:/ So I decided to create a more feminine strong female character with a more complicated and somewhat “unlikeable” personality at times:)
Speaking of fixing up troupes:) go to http://tvtropes.org/ really fun place to get story ideas or just fandom dafuq out:3
Anyhow hope this helps