Facebook Instant Articles support is integrated via official Instant Articles for WP plugin (built by Automattic), available for free download in the WordPress.org repository. You can also install it directly from the WP admin ▸ Plugins ▸ Add new.
After installing the plugin configure Facebook Instant Articles in WP admin ▸ Instant Articles.
After creating / editing post, the plugin automatically will send post’s content converted into Instant Article to Facebook. Status of this action can be seen in post edit page via Facebook Instant Articles meta box. For more information regarding configuring plugin and process of submitting posts to Facebook please visit the plugin’s page here