To create a new Article:
Navigate to an Article which is to be the parent or sibling of the new Article
From the middle green button at the top of the page, select whether you'd like the new Article to be a child of the current Article ("below") or if you'd like it to be a sibling of the current Article ("next to")
From there give your new Article a name, contents and a summary
Click "Create Article"
Once your Article has been created, you will often make it accessible to others by linking to it from other Articles. To do so navigate to the Article from which you want to link, then create a link to your new Article.
If you do not see the green buttons at the top of the page, then you don't have permission to create new Articles in the current Article Collection.
Two notes:
To add images, links and attachments, save your Article and then go Edit it. These options are available in the editor. (This discrepancy will be resolved soon!)
The "slug" is the path to the article. Typically the default slug can be left alone.