I make an outline of what I'm thinking of, then I start throwing in some characters who I want in it, especially if there's OCs, whom I immediately start designing for reference. But mostly I have to sit on the idea for a bit to see if I'm amused or interested enough by it to start writing down dialogue. That's usually what I jot down first is a string of dialogue that I fill in the gaps as I go along. Any scenes that come to mind of what I'd like to see in the story I jot down a quick summary on its own separate page of the document until I reach that point to put it in the story proper.
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Actually, I kinda don't really pay much attention to what I do when I plan things out. The above's typically what I do even as I'm going along, but ideas/plot-bunnies just come to me at random (usually in the shower or as I'm listening to music/out on a walk) that I let fester as I try to piece together what I want. For the most part, I come up with what I want the ending to be, the "moral of the story" so I have a good idea of what my goal will be. If the idea sticks with me and continues to grow within the week or two, that's pretty much a good indication that I like the concept enough to want to finish it (someday... eventually). To keep myself interested, I'll even go so far as to sketch out scenes when I'm not near my computer.
Given that I have a bad habit of procrastinating and not work on stuff for months or years on end (I rarely delete documents), if it's something that really wants to be written, it's gotta grab my full attention, and I have to basically be working on it for months at a time in order for me to get it in my thick skull that I'm being serious about it.