These instructions assume you are building a BOFU ad to drive content to a landing page. There are three parts to this.
First, create a square featured image. You do that the very same way you create any other image, then upload it to WP, and assign it to your landing page.
Second, if you "publish" a square ad to your Facebook page, it will get cropped to 1.9:1, so don't do that! You'll want to use a FacebookAds venue (so you're promoting) but UNSELECT the "publish" option. This used to be called a "dark post" but the modern term is the so much more compact name "promoted non-published post". sic
Third, in ResultFlow, you will edit the "meta-data" on the post to add the CTA (call to action).
Here's a video demonstration: