Add you! Add your username on the participants list of the current edition Select an article! You can find suggested articles on the current editionspage Write, translate or improve it! Start writing the article from scratch, translate it from other languages, or improve an existing one. Check it! Add the new or improved article to the results list of the current edition.
The campaign is cross-wiki - besides writing or improving Wikipedia articles - you can also contribute by creating new Wikidata items, uploading on Commons photos with free licenses or adding categories, writing an article on Wikinews or adding a new entry on Wikiquote, encouraging someone to become a Wikimedian or give some help to a new editor. Every single action is valuable.