Board Thread:Wiki news and announcements/@comment-26431461-20160905200618/@comment-2234967-20160905223751

If you list categories in the category section on the edit article page, they'll be auto-added to the article when you send the edit through.

Unless you're talking about how to actually make category pages, which is pretty much the same as making regular articles. Just click the category name at the bottom. If there's no content for it, you can create the page and add it in.