In-article ads will display an ad after the first 3 paragraphs and then every 5th paragraph after.
If the article does not reach 3 paragraphs the ad will not display.
Create a new ad object named in-article, use 300×250 for dimensions for best display across devices. But you can use the responsive size mapping and use 728×90 for the larger size and 320×50 for the mobile size.
Navigate to Advertisements > ad settings and select the new in-article ad object from the drop down under in-article ad slot and click the update button to save.
The in-article ad is a global setting but it can be disabled on an article level.