Create a newsletter issue or publication edition as described in the KB articles: 

How do I create an eNewsletter issue? - stop after step 4 (optional)

How do I create a new edition or issue of a publication? - stop after saving the edition (optional)

In the Article Editor, access the Newsletters field. By default it appears in the Show More Fields dropdown but you can pin it to the right column so it is always displayed by default. 

Click in the Newsletters box to access a list of upcoming Newsletters and Issues. The list will only show Newsletters with Issues created within the last 3 days by default. See Updating System Settings to adjust this timeframe.

Select the newsletter issue you want the article included in.

Access the newsletter issue to review articles that have been added from the article editor. The most recently added article will appear at the top of the list by default.

Publication editions work in the same way as Newsletter issues with one difference. Articles are always associated with a Publication Edition Department, so there is one more dropdown layer to navigate.

Editions will appear in the list for 30 days by default. See Updating System Settings to adjust this timeframe.

To reduce clutter, only recently created issues/editions will appear in the dropdown list for selection. 

By default, a newly created issue will appear for three days. After three days, the newsletter issue will fall of the list and won't be relevant because the issue has presumably been completed and sent.

If your business rules have you working on newsletters longer than three days, you can control the timeframe in the article.days.to.show.newsletter.issues system setting.

Likewise, a newly created publication edition will appear for 30 days by default. This can be increased or decreased in the article.days.to.show.publication.editions system setting.

Contact your PM if you need help editing system settings.