You'll need to first sign up with Outbrain. Outbrain currently offers you both text and thumbnail variations for displaying page recommendations, and two options for displaying feedback (shown below).
The method for adding either variant is the same. After choosing your options, and selecting Squarespace as your platform, Outbrain will provide you with the code that you can add to your site.
You can add this from [Website Management > Structure > Website Settings > Code Injection]. Add the code to the Pre-Header HTML Code injection point. Code Injection is available on the Unlimited site package.
To verify ownership of your blog with Outbrain, add the second block of code they provide to the Pre-Header HTML Code Injection point.
Note: The journal page module must be set to visible for the public audience.