Add the article to WP:FA in the appropriate category and update the tally at the top of the FA page. If the article is a former featured article that has appeared on the main page, add the appropriate syntax to its entry so that it won't accidentally appear again.
If it is a former featured article, update WP:FFA. Move the article from its category to the re-promoted category at the bottom. At the top of the page, adjust the tally of re-promoted articles.