WordPress custom fields allow you to add extra content to your posts in the form of key-value pairs. Here’s a guide to using WordPress custom fields to add extra content to your posts:

1. Install and activate the Advanced Custom Fields plugin: Advanced Custom Fields (ACF) is a popular plugin that makes it easy to add custom fields to your WordPress site. To use it, you’ll need to install and activate the plugin on your site.
2. Create a custom field group: In the WordPress admin, go to Custom Fields > Add New to create a new custom field group. Give your custom field group a name and choose the “Post” post type to apply it to.
3. Add custom fields to your group: Click the “Add Field” button to add custom fields to your group. You can choose from a variety of field types, such as text, number, file, or image.
4. Customize the display options: You can customize the display options for your custom fields by clicking the “Location” tab. Here, you can choose which post types and post statuses your custom fields should be displayed on.
5. Add custom field values to your posts: Once your custom field group is set up, you can add custom field values to your posts. To do this, go to the post editor and scroll down to the “Custom Fields” meta box. Here, you can add custom field values by entering a key and a value.
6. Display custom field values on the front-end of your site: To display custom field values on the front-end of your site, you’ll need to use the get_post_meta function in your theme’s template files. For example, you might use the following code to display a custom field value called “subtitle” in your single post template:

<?php
$subtitle = get_post_meta( get_the_ID(), 'subtitle', true );
echo $subtitle;
?>

Custom fields can be a useful way to add extra content to your WordPress posts and create more flexible and customizable content. By following this guide, you should be able to use custom fields to add custom content to your WordPress posts.