To create an article in an existing category that is in a level that is setup to show administrative edting prompts the easiest way to create a new article is to:

1. Login to the Admin Center as an administrator
2. Navigate to the category as a normal site visitor would. 
3. Look for the Edit Content link and click on it.
4. Make your changes (usually in the Brief)
5. Click Update. 

If the site level hides administrative prompts right click on the content area to reveal the Edit Content link. 

The manual way to accomplish this is to:

1. Login to the Admin Center as an administrator. 
2. From the Content segment select the Content button.
3. From the Category menu on the right select the category you wish to edit. 
4. Find the article you wish to edit or even use the search tools to find it. Click on the article you wish to edit. 
5. Make your chagnes (usually in the Brief) 
6. Click Update.