Now that you've hit the "create page" button, you'll face an edit window box with some odd-looking syntax on it. Don't edit or remove any of it for now; after you save the page, all of it will magically be transformed into nice-looking templates.
The first thing to do is cite the sources you'll use. At the bottom of the preloaded page, find the following:
*{{source|url=
|title=
|author=
|pub=
|date=}}
Copy/paste this template several times so that there's one for each source you're using. Copy the URL of each source into the "url" tag, its headline into "title", and insert the story's publisher into the "pub" parameter. If the web page indicates the person who wrote the story, add it under the "author" parameter. Also include the date when the source published its story under "date=".
Make sure to only cite the sources necessary to back up all information. Don't link to sources with info that isn't found in your article - that's just an unnecessary workload for the reviewer who will go over your article (more on that later).
Once you're done with sources, hit "Save page" at the bottom of the edit box to save your work. Then, click "edit" at the top of the page to return to the edit window. You're going to write the actual content of your story. You should write the text between the {{date}} and {{haveyoursay}} templates near the top of the page.
When writing, use "reverse pyramid structure" - that is, most important information at the beginning, with details being provided later on. Keep your paragraphs fairly short, splitting them once every few sentences to make it easier to read/digest. Your story should have at least three paragraphs, ideally more than that if extra information is available.
Use a formal, journalistic tone when writing; avoid colloquialisms and don't be sensationalist - we're not a tabloid. Keep in mind that all stories must be neutral; don't promote one point of view over another and present all sides of an argument as fairly as you can. Avoid inserting your personal opinion; just state everything factually.
Try not to copy or paraphrase things too closely from the sources you use (aside from direct quotes from people); write things in your own words. We don't want to infringe copyright.
The style guide lists all the ins and outs of proper writing style. It's very long, and you don't need to know every detail of it to write well, but skim through the more important points if you have time, to get a feel for what style we use.
Once you've finished writing, simply replace the {{develop}} tag at the top of the page with {{review}} and someone should be around to look over it. If it gets approved and published, great! The story will appear on the front page, as well as Google News, our Twitter feed, and multiple RSS feeds.
If it doesn't meet publishing standards for some reason, the article will be bumped back to developing status, with a comment on the corresponding talk page saying what's wrong. When you feel you've addressed the listed concerns, feel free to add {{review}} once again.
