1. Create a new snippet and select Post content as snippet position. tab
2. Choose after how many paragraphs do you want to load, you can add 0 to load the script before the post content, -1 to load the script after the post content or any value, our system avoid to break the post content.
3. Select which custom post types you want to include or exclude (depending by the chosen custom post type match value)
4. Put your banner code and click on Save button.
Here a videotutorial that will help you to display a banner inside the WordPress post content