First, pick a conflict: Man vs. Man, Man vs. Circumstance, or Man vs. Self. I know there are over thirty tropes, but these three will suffice. Next, I write a little column of three abbreviations...
The first is what the main character wants to achieve. I think this can be anything, though some would suggest it's important that the reader be able to relate to the goal.
The second is "obstacle," that is, the primary thing in that person's way. There could be several obstacles, and many of my short story ideas have two sets of these points: one for the Man vs. Man storyline and one for the Man vs. Self parallel thread.
The last is very important because it seems to be the hook in good fiction writing. It represents what happens if the person fails in their little quest, and the repercussions must be disastrous. It's not just a matter of disappointment ("Oh, I really wanted that slice of cake."), it's a life-changing result ("Well, without my job, there goes my car payment and child support."). It's that tension from the high stakes that makes it really gripping.
Obviously, any of these can be tweaked for specific effect. Once again, I just want to impress this is simply a brainstorming exercise, not to be argued with over finer points and hair-splitting. It's more important to get the idea out on paper and actualized, rather than quibbling over how its reception should present.
This is my takeaway after reading the first few chapters of Practical Short Story Writing by John Paxton Sheriff. Despite it having been written in 1995 (rereleased in 2000) and constructed in England (I don't recognize any of the literary magazine it cites), it's a wonderful spark of inspiration. I've been struggling with my writing so any impetus is welcome. I was out on the town when this inspiration struck but happened to have some junk mail in my pocket: I tossed the contents and meticulously deconstructed the envelopes to gain as much writing space as possible, and I filled them with a dozen short story ideas.
And this topic may sound like a deviation from my normal focus, and it is, but whatever. I haven't written anything in a long time, and I would contrive to laud writing exercises as useful for personal correspondence, as who doesn't like a good storyteller?