First you need to visit Facebook Instant Articles website and click on the sign up button to get started.
Sign up for Facebook Instant Articles
Facebook will now ask you to select a page. Here you need to select the Facebook Page for your website.
After that check the box to agree with Instant Articles terms and then click on 'Access Instant Articles Tools' button.
Select your Facebook page
This will take you to the publisher tools on your Facebook page, which will now have an instant articles section.
First, you need to prove the ownership of your website by claiming your URL.
Claim your website URL
Scroll down a little to the Tools section on the page and click on the 'Claim your URL' to expand it. Facebook will show you a code snippet.
Meta tag to claim ownership of the URL
You need to copy the code and insert it into the <head> section of your WordPress site.
There are two ways you can add this code to your website.
You can edit the header.php file in your child theme and paste the code just before <head> tag.
But if you are not using a child theme, then you can use Insert Headers and Footers plugin. For more details, see our step by step guide on how to install a WordPress plugin.
Source: http://www.wpbeginner.com/