General advice up front:
Be careful about setting the language of the text to the right dialect of English. If the article is not about a topic tied to a particular region, use the dialect of the first non-stub version of the article.
See for a handy chart showing different spellings in different dialects.
Check the talk page before editing dialect-specific spelling. Some talk pages have banners near the top that indicate which spelling is used throughout the article.
Keep spelling consistent in a given article (excepting direct quotes), and write in ways understandable in all dialects when possible.
See for detailed advice.
The easiest way to spellcheck a single article is to use the built-in spellcheck of your web browser.
The default setting in Microsoft Office is with spell and grammar checking on, so just hit the "Edit" button for your article, copy the raw article source, paste it into a new Word or Writer document and follow the red (spelling) and green (grammar) markers and correct mistakes as necessary. If checking is not automatic, you may have to go to the menu Tools -> "Grammar and spell checking" or some such. When satisfied, simply copy and paste the text back into the text box in the browser window, check that any UTF-8 characters are still working as before by clicking "Show changes" and if satisfied, finally click "Save page" as usual.
If you have installed the anti-vandal tool in , you can use the live spellcheck to identify misspellings as they happen (recent edits). This is not a comprehensive spell check – it only points out the most commonly misspelt words. And it will not spellcheck text that an editor is adding to an article (for that, see the web browsers section, above).
See .