Wow Gib. Someone really pissed in your Wheaties this week didn't they? You want me to kick someone's ass? Why so cranky?
Anyway, Mari you should keep in mind that Gar and Gib are both good writers who operate on a completely different playing fields as far as their process goes. Which should underline to you the old adage that there are indeed many ways to skin a cat.
From looking at the problem you presented my first instinct would be to look at the magazine. What type of readership does it have? What is the overarching theme of the periodical? What are it's goals and ambitions? That's where I would start because I feel that in this instance, when you are being contracted for a specific audience it is more important than usual to give the audience what it wants.
With Shizentai you have this long form, recurring character story told in a very Asian flavoring. You wouldn't offer that type of thing up if you were asked to do a comic for "Popular Mechanics" now would you?
The magazine I write for is a Fantasy Literature mag so naturally anything having to do with Fantasy is in the money spot but other types of speculative fiction are encouraged too. The trick is to give the audience what it expects.
Once you figure that out then you have a beginning point. One shots are pretty easy in my opinion. So easy that I don't get a whole lot of satisfaction out of making them. Take a look at DC Comics "Elseworlds" series where they take familiar super heroes and put them in different times, places, even dimensions or planets. Those were always some of my favorite comics and they were almost always one shots.
I have one called "In Darkest Knight" where Bruce Wayne gets the Green Lantern ring from Abin Sur instead of Hal Jordon and becomes a sort of Batman/Green Lantern hybrid. There's another one where Kal El comes to Earth during the Revolutionary War and ends up fighting for the British. Needless to say things didn't go too well for the Americans in that revolution.
I recommend you pick one up if you're interested in the format. They really do a wonderful job of establishing the hero and telling a tale in a completely new world; all in one 30-40 pg mag.
They really are easy though. The trick, to my mind is just shutting off the possibilities in your mind. Whenever I'm writing the long form stuff, even if it's just one three panel strip, I'm thinking about all the things that have come before and all the possibilities and probabilities that can come in the future and how that one strip will marry into all that.
I'm thinking "oh if this character does this in this strip I can have him do this in two years and it will explain why he doesn't like sea bass" or something equally crazy. But when you are writing one shots you just have to let all those possibilities go. Write the story and get used to hitting all your big plot points hard and often (the reflex for writing long form is to build up to those moments but when you are doing one shots you just have to let it fly). And don't bother thinking about what the future may bring. If your characters survive the experience and you get an idea to bring them back or use them again in another project then great. But for the purposes of this one shot, this is their whole existence. They could all drop dead after the last panel but as long as you've resolved all the questions you raised in act one your readers will be satisfied.
And that brings me to basic structure. Most of my comics are written in 3-4 panels. I alway try and follow the three act format as closely as I can.
1) Present your protagonist with an internal and external problem. (Put your main character up a tree, reveal he is afraid of heights)
2) Make the problems worse. (Have your antagonist throw rocks at him forcing him to climb higher, increasing his fear of heights and making it more difficult for him to get down).
3) Get your Protagonist out of the tree (thus showing he has defeated the external problem of being up a tree and conquered his internal problem by getting out of the tree without giving in to his fear of heights).
These basics hold true on a macro and micro level. So if you are writing a scene that involves one page that scene should have a first, second and third act and in turn be a part of the larger structure of the overall work by fitting in to the first, second or third act of the entire piece.
Probably the best thing you can do though is pick up a book of really good quality short stories. In fact, if you like fantasy the magazine I write for
Black Gate has some wonderful short fantasy fiction in it and the PDF download versions of the issues are very reasonably priced below print copies and with of course, no shipping charge.
Because one of the hardest things to do with one shots is world building. And once you have that down the rest is pretty easy. Short stories, good ones anyway, are great at showing you how to build a world and establish the rules of that world in a very short period of time. And that should help with your one shot writing.
Shorter works like one shots and short stories are about quality over quantity. You have to think a little bit differently to get the story on the page. Good luck though. Let us know how it works out.