As with the image composition, I start with the point of focus. Usually this is both a physical point and an action; either the action is the point of focus, or the action bridges between two points of focus. If the former, I figure out what’s moving to create that focus (in a fight, this is usually both the attacker and the defender, and it really doesn’t matter who’s which), and then use them as my point of origin. What are they? What sets them apart from the rest of the fight? What are they beginning to do? I also, particularly if it’s a fight in which I control both sides and don’t have to worry about where the dice land, come up with my optimal ending image.
Then there’s the motion. Sometimes it’s one basic moment of activity—one attack, one parry, one clever maneuver, one instance of a character getting where she shouldn’t be. If it is, it’s that that I focus on—as if it were a movie, I give a blurry, detail-minimum description of the actions leading up to the moment (basically, no adjectives beyond direction and maybe color), then a snapshot of the moment as it takes place in as minimal but specific detail as I can (again, centered around the point of dominance), then finish with what the moment should leave behind (either explicitly if I’ve got the dice, or in terms of “(hopefully) this happens” if I’m trying to present an action in someone else’s game. If it’s a longer-running action—maybe long motion, or some sort of multistage process, I’m likelier to present it as a series of snapshots of the important parts. It’s rather like trying to novelize a comic book, really, down to the simplest form: a whole lot of intersection character verbs nouns, bridged by prepositions. Character ducks behind noun in order to verb other noun. Other character verbs—another noun, or perhaps the first character—in response. One succeeds. One fails. Sometimes both go off at once, and things get a touch awkward. Action action action consequence consequence. I think I may need to come back to this with a diagram.
Once I’ve done that, I tighten up my phrasing. Action scenes being action scenes—particularly these ones where the defining part is supposed to take only a heartbeat or two—I try to keep my language sparse. It’s one picture, after all. I shouldn’t need more than two colors (though I have been known to count things like light-scattered-into-rainbow-by-crystals as one color), I definitely shouldn’t need more than one adverb, and anything I can do by using the most precise possible word for the job, or a single simile or metaphor that covers as much of the picture as possible, I do. If I can find words that play to the mood I want, so much the better—if not, I add just enough background detail to get it across.
How long this process takes varies. On a good day, I can do an action in five minutes, sometimes less. It takes longer when I’m doing all the work; when I last ran a fight in which all five (later six) of the combatants were my characters and the one PC was staying out of the way, I averaged about eight minutes per post, one to two actions each. Part of the key is practice, and part is character knowledge—know both, do it a lot, and it becomes a lot easier.