Writers are always talking about plot, themes, pacing, voice and so forth, but I rarely see anyone discuss scene composition. I have very little idea of how other writers compose scenes, either, other than the prep work they put into thinking about them and doing notes and outlines and other ways of getting ready to write them. Actually writing scenes seems to be the hard part.
I've talked about my before-writing prep for scenes, but here is what I do to write any scene in any book in any genre:
1. I open with action or dialogue (or as close to it as is possible.)
2. I write straight through the scene, layering action with dialogue and working in whatever else I need to match what I saw in my head when I visualized. The research and notes and particulars from the novel notebook pop up in my head whenever I need a detail, but mostly I'm a voyeur scribbling down what I see.
3. If I get stuck, and it's usually on setting, which I hate, loathe and despise above all other components of the book, I'll put a rewrite marker in brackets like this: [***description of Central Park in winter***] and move on.
4. I do not back track or re-read anything I write while I'm writing new material. Ever. No exceptions (this is a huge time-waster.)
5. When the scene is done, I take a break (usually a cup of tea or a walk around the house) and then move directly on to writing the next scene.
If I'm not writing with the voice recognition software, I'll sometimes recite dialogue lines out loud as I type it to get a feel for the flow of the words (yes, I'm told this seems as amusing and ridiculous as someone who sound out words while reading. Tough. It helps.)
I also don't worry about descriptive words or if I'm showing enough of the setting in the scene. What hits the page is usually enough, but if I think I need more I'll add it during editing. I think this might be a key problem for writers who get hung up while writing a scene -- they hit something they don't want to write, aren't ready to write, can't express correctly, etc. and sit there staring at the screen. I suggest putting in a rewrite marker (see #3) and moving on.
I've gotten into the habit of backing up the document after I finish a scene, even though Word does a auto-backup every couple of minutes. This is more like a nervous habit, from a lifetime of living in the lightning capital of the U.S., but it also helps me move on. I have no idea why, but it does.
More info: Holly Lisle's Scene-Creation Workshop -- Writing Scenes that Move Your Story Forward has some excellent ideas to help you out with scene composition.
Don't know what the elements of a scene are? Check out Writing Fiction: A Beginner's Guide Part 7: Scenes and Half-Scenes (this is creative writing for teenagers article series, but a good breakdown just the same) or Vicki Hinze's article Elements of a Scene.
Randy Ingermanson's Writing the Perfect Scene.