How do I create an rssfeed from the contents on this page?

You can take a look at

ABSPATH . WPINC . 'feed-*.*';

to see examples. WP core has feed tempaltes for the following protocols:

Atom
RDF
RSS
RSS2

When writing a plugin, you can take the following quick test plugin as base. It adds a feed name custom-feed to the list of feeds.

<?php 
/* Plugin Name: (WPSE) #172796 Custom Feed */

add_action( 'init', function()
{
    add_feed( 'custom-feed', function()
    {
        /** @var \WP_Query $wp_query */
        global $wp_query;

        // Custom conditions when to output the feed - and when not
        if ( 'foo' !== get_query_var( 'name' ) )
            return;

        load_template( plugin_dir_path( __FILE__ ).'templates/feed-rss2.php' );
    } );
} );

To get a quick look at the default feeds, simply dump them

var_dump( $GLOBALS['wp_rewrite']->feeds );

When you activate above test plugin, you should find custom-feed as last entry in the array.

Inside any callbacks you should check if you are actually working on the new custom feed:

$wp_query->is_feed( [ 'custom-feed', ] );

To get a valid feed, you will have to set a header. Core does it like the following:

header( sprintf( 
    'Content-Type: %s; charset=%s',
    feed_content_type( 'rss-http' ),
    get_option( 'blog_charset' )
), true );

Then there’s the actual XML opening tag:

echo '<?xml version="1.0" encoding="'.get_option('blog_charset').'"?'.'>';

after that, there’s a place to allow plugins to hook in:

do_action( 'rss_tag_pre', 'rss2' );

Finally there is the <rss version="2.0" ... tag opening and another hook to add custom namespaces

do_action( 'rss2_ns' );

The list goes on and on. The best you can do is to take the core files as blueprint for your own feeds (copy/paste them to your plugin and alter the output).

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