Action Details
Requirements and how-to articles for Pinterest's action:

Rich Pins are a type of organic Pin format that provide more context about an idea by showing extra information directly on the Pin.

You need this to get it done:

Meta Tags
Add metadata to the page on your website which you want to create a Rich Pin for (it is highly recommended to use the "Yoast" SEO plugin to make this easier.)
Validate URL
Enter your page URL into the Rich Pins Validator to check the metadata is correctly set up.
If there are no problems with your page's metadata (or if there are any, they have been corrected) select from one of the three options to indicate how you marked up your page (HTML Tags, oEmbed or Shopify.) Then click "Apply now" and wait for Pinterest to approve your Rich Pin. Once one link from your site has been approved, your entire domain will be available to use for Rich Pins without needing to validate each page.
Requirements missing or found errors? Report

How-To Articles
A list of useful links for you:

Pinterest Help Centre - Rich Pins
Official information on Rich Pins.
Pinterest Developers - Rich Pins Validator
Official step-by-step guide on how to validate and apply for Rich Pins, with the URL Validator at the bottom of the page.
Link missing here or found errors? Improve How-Tos