The first time I get an idea for any story, I write down its summary first. I find that when I'm writing that down, all other ideas jump out at me, and I scribble little notes on a notebook. I usually write the ending first because I tend to have a very clear picture in my mind of how I want the story to go. Once I'm satisfied with that, I write the beginning, but I prefer to let the middle bits write themselves - I just see where they take me. I carry around a little notebook with me out of habit, so if I get a burst of inspiration for the part I'm working on, I can jot it down. I also tend to have "conversations" with my characters in my head, so I can write them with a specific personality in mind.

From the above prompts, I think I'll pick "Harry and Draco didn’t meet in Hogwarts. Instead, they meet at the Quidditch World Cup as rival seekers." because I've never done an AU before. I'd begin by thinking of the direction I want this to go in and plan out the ending - is it happy? hopeful? open-ended? - and then I'd work on the details. Which teams do they play for? If they didn't meet in Hogwarts, does that mean one of them went to a different school? Next would be the meeting - what game do they meet at? The first match, or finals, or even a glimpse of each other at practice? Who would win, and how would they continue on from there (Would someone get asked out already, or would they keep bumping into each other after that? Would there be attraction from the beginning, or would someone have to be transferred to the other team first?). There are endless possibilities!