I am assuming that you have an AO3 account and that you are logged into it. (If you do not have an AO3 account and you would like one, let us know in the comments of this post! We have extra invite codes!)
What you will be doing is posting your story to a collection. The AO3 has several FAQs on works in general and how to post to a challenge. The tutorial on challenges is very relevant; in fact, that's what you'll be doing. The only differences are that in the kind of challenge the tutorial is discussing, your story would (a) have a recipient and (b) be hidden by the moderators until a later date. We the mods are not planning on running the collection hidden like this; your story will be live on the site and readable as soon as you post it, so PLEASE only post on your assigned date.
(Why are you posting to a collection? Well, if all the Big Bang stories for this year are gathered together, it makes them much easier to find and read. That's pretty much what collections are for.)
SPECIAL NOTE FOR PEOPLE WHO ARE EXPERIENCED AO3 USERS AND JUST WANT TO KNOW WHAT COLLECTION TO POST TO: Pros_Big_Bang_2011.
UPDATE FOR 2012: THE NEW COLLECTION IS Pros_Big_Bang_2012.
For anyone who wants them, step-by-step instructions:
What you want to do is go to the page for the CI5 Box of Tricks 2011 collection. You'll know you're there when you see the banner of the Lads across the top!
NEW FOR 2012: You want to use the 2012 collection instead. All other instructions remain the same!
You want to click on the button that says "Post To Collection," on the right:
I have indicated this button for you.
This will take you to the Post New Work page, specially ready for your story:
Basically, fill out the fields of the form as indicated. You have to choose a rating from the dropdown, but that rating can be Not Rated. Similarly, you have to check at least one box under "archive warnings," but that box can be Choose Not To Warn.
All the text boxes of the form will attempt to autocomplete. So start typing "The Professionals" under Fandoms and it should be selectable (either click on it or arrow down to it and hit return) by the time you hit "The P," in my experience. If you have a second fandom (say, if your story is a crossover), you can add a comma (everything is separated by commas) and the name of your other fandom(s).
If you make a mistake typing any of these tags and add something you don't want to, clicking the red X to the right of your completed tag will make it go away.
Everything else in the header is optional... but it's a really good idea.
Category: Check the appropriate box(es). I expect many of you will want "M/M." :)
Relationship: Start typing "Bo" and the dropdown will offer you "William Bodie/Ray Doyle." (AO3 lists relationships alphabetically by last name, usually.) If there are any additional pairings you want to list, separate them by commas. If they don't autocomplete, don't panic; it probably just means no one's posted them to the archive before -- you can type in new ones yourself!
Characters: List any important characters whose presence you would like to inform the reader of, separated by commas.
Additional tags: This is where you can list... well, anything you want. A lot of people (like me) use it for story elements and genres. Angst, Hurt/Comfort, Alternate Universe, Undercover As Gay, First Time, and so on and so forth. Readers can search the archive based on these tags. As someone who regularly uses the archive to do things like read all the stories tagged with Werewolves, I find this kind of thing helpful. (If you're not sure what to put, check out this list of common tags for some ideas.) Again, this is optional! But fun!
Work Preface section: Put your title, summary, etc. in the appropriate boxes. There are also checkboxes if you want beginning notes or end notes -- once you click one of those, a box to type your notes in will appear. If you aren't linking to your art within the text of your story, you might want to add a link in the notes.
Associations section: Important!!! This is what puts your story in the Pros Big Bang collection. Where it says "Post to Collections/Challenges," you want to make sure it says Pros_Big_Bang_2011, as in the following picture:
If you followed my directions and got to this page by clicking on the button from the CI5 Box of Tricks 2011 home page, this should already be filled out for you.
(If you got here some other way, accidentally deleted the words, or something else unforeseen happened, and the words aren't here, you can just type them in. If for some reason you managed to post the story without it, don't panic; you can always edit the story and add it, and then your story will be in the collection. In fact, you can edit absolutely anything later and fix it!)
Everything else in the Associations section is probably irrelevant for our current purposes.
Work Text: Your story, as you can probably guess, goes in the box -- post in either rich text or HTML. I have never tried the Rich Text format myself, but as for the HTML, I can tell you that it works exactly like posting to LJ. (Just as with LJ, you do not need to include paragraph tags and can just use a carriage return between paragraphs. I should mention, however, that it automatically inserts paragraph tags for you once you submit it, in case you need to look at the code again.)
I should also mention that the AO3 is capable of handling a much larger amount of text (500,000 characters) in a story than LJ (approximately 65,000 characters = about 10,000 words, maybe less). Therefore unless you are very verbose your entire story will likely fit within a single post. (If it doesn't, that's what chapters are for.)
You will probably want to hit "Preview" rather than "Post without preview" and make sure everything looks okay before hitting the final Post button at the bottom. (Though you can edit it later -- the edit button is accessible from either the story itself or your user homepage.)
If you'd like to practice posting things, you probably have other stories you can upload. Anything from last year's Big Bang goes in the Pros Big Bang 2010 collection. Stories that are not part of any collection can be posted using the "Post New Work" link in the upper right corner of nearly every page on the site.
Artists and vidders: You can also post your art to AO3, if you'd like to take advantage of the tagging features. You will have to host your art elsewhere and add an HTML link to the art/vid in the "work text" section. (There is a guide here for vidders.) (Even better, the archive will let you associate your work with your author's story; doing this is easy and basically involves checking the "inspired by" button on the posting form and providing a link to the author's story.)