Editors' Guide: Related Stories
To relate one story to another, so that a link appears in the sidebar under Related Stories ...
On the story Publish page, you'll see a tab called Related Stories. Click it.
Type some headline keywords in the "Search Entries" field to find the story you want to add. Choose a filter to limit your query to articles in a specific section. Click Search.
When the story you want appears below click the blue button 
to select the story and move it to the second column.
This creates a sidebar link on both stories -- the stories being linked, and the one being linked to.
To make this story the one listed first (if there are others), click the radio button beside the headline in the second column.
To remove the story from the related story list, click the circle in the second column with the red minus sign beside the headline.
Click "Update" (not "Quick Save") when you want to save. "Quick Save" (for simple text edits on the page) will not make the addition.