There’s three main steps to create your custom RSS Feed.

1. Register your feed in functions.php

   add_action( 'after_setup_theme', 'custom_rss' );
   /**
    * Register custom RSS template.
    */
   function custom_rss() {
   	add_feed( 'custom', 'render_custom_css' );
   }
   /**
    * Custom RSS template callback.
    */
   function render_custom_css() {
   	get_template_part( 'feed', 'custom' );
   }
   

2. Create your template file feed-custom.php in your theme folder.
   You can copy and paste from wp-includes/feed-rss2.php to make things easier and make changes something like the following – I fetch post with post_type is ajde_events and is it a featured event:

   <?php
   /**
    * RSS2 Feed Template for displaying RSS2 Posts feed.
    *
    * @package WordPress
    */
   
   header('Content-Type: ' . feed_content_type('rss-http') . '; charset=' . get_option('blog_charset'), true);
   $more = 1;
   
   echo '<?xml version="1.0" encoding="'.get_option('blog_charset').'"?'.'>';
   
   /**
    * Fires between the xml and rss tags in a feed.
    *
    * @since 4.0.0
    *
    * @param string $context Type of feed. Possible values include 'rss2', 'rss2-comments',
    *                        'rdf', 'atom', and 'atom-comments'.
    */
   do_action( 'rss_tag_pre', 'rss2' );
   ?>
   <rss version="2.0"
   	xmlns:content="http://purl.org/rss/1.0/modules/content/"
   	xmlns:wfw="http://wellformedweb.org/CommentAPI/"
   	xmlns:dc="http://purl.org/dc/elements/1.1/"
   	xmlns:atom="http://www.w3.org/2005/Atom"
   	xmlns:sy="http://purl.org/rss/1.0/modules/syndication/"
   	xmlns:slash="http://purl.org/rss/1.0/modules/slash/"
   	<?php
   	/**
   	 * Fires at the end of the RSS root to add namespaces.
   	 *
   	 * @since 2.0.0
   	 */
   	do_action( 'rss2_ns' );
   	?>
   >
   
   <channel>
   	<title><?php bloginfo_rss('name'); wp_title_rss(); ?></title>
   	<atom:link href="<?php self_link(); ?>" rel="self" type="application/rss+xml" />
   	<link><?php bloginfo_rss('url') ?></link>
   	<description><?php bloginfo_rss("description") ?></description>
   	<lastBuildDate><?php echo mysql2date('D, d M Y H:i:s +0000', get_lastpostmodified('GMT'), false); ?></lastBuildDate>
   	<language><?php bloginfo_rss( 'language' ); ?></language>
   	<sy:updatePeriod><?php
   		$duration = 'hourly';
   
   		/**
   		 * Filter how often to update the RSS feed.
   		 *
   		 * @since 2.1.0
   		 *
   		 * @param string $duration The update period. Accepts 'hourly', 'daily', 'weekly', 'monthly',
   		 *                         'yearly'. Default 'hourly'.
   		 */
   		echo apply_filters( 'rss_update_period', $duration );
   	?></sy:updatePeriod>
   	<sy:updateFrequency><?php
   		$frequency = '1';
   
   		/**
   		 * Filter the RSS update frequency.
   		 *
   		 * @since 2.1.0
   		 *
   		 * @param string $frequency An integer passed as a string representing the frequency
   		 *                          of RSS updates within the update period. Default '1'.
   		 */
   		echo apply_filters( 'rss_update_frequency', $frequency );
   	?></sy:updateFrequency>
   
   	<?php 
   	$args = array(
   			'meta_key' => '_featured',
   			'meta_value' => 'yes',
   			'post_type' => 'ajde_events',
   			'post_status' => 'publish'
   		);
   		$the_query = new WP_Query( $args );
   	?>
   	<?php do_action( 'rss2_head'); ?>
   	<?php if ( $the_query->have_posts() ) : ?>
   		<?php while ( $the_query->have_posts() ) : $the_query->the_post(); ?>
   			<item>
   				<title><?php the_title(); ?></title>
   				<link><?php the_permalink_rss() ?></link>
   				<comments><?php comments_link_feed(); ?></comments>
   				<pubDate><?php echo mysql2date('D, d M Y H:i:s +0800', get_post_time('Y-m-d H:i:s', true), false); ?></pubDate>
   				<dc:creator><![CDATA[<?php the_author() ?>]]></dc:creator>
   				<?php the_category_rss('rss2') ?>
   
   				<guid isPermaLink="false"><?php the_guid(); ?></guid>
   				<?php if (get_option('rss_use_excerpt')) : ?>
   						<description><![CDATA[<?php the_excerpt_rss(); ?>]]></description>
   				<?php else : ?>
   						<description><![CDATA[<?php the_excerpt_rss(); ?>]]></description>
   					<?php $content = get_the_content_feed('rss2'); ?>
   					<?php if ( strlen( $content ) > 0 ) : ?>
   						<content:encoded><![CDATA[<?php echo $content; ?>]]></content:encoded>
   					<?php else : ?>
   						<content:encoded><![CDATA[<?php the_excerpt_rss(); ?>]]></content:encoded>
   					<?php endif; ?>
   				<?php endif; ?>
   				<wfw:commentRss><?php echo esc_url( get_post_comments_feed_link(null, 'rss2') ); ?></wfw:commentRss>
   				<slash:comments><?php echo get_comments_number(); ?></slash:comments>
   				<?php rss_enclosure(); ?>
   				<?php do_action( 'rss2_item' ); ?>
   			</item>
   		<?php endwhile; ?>
   
   		<?php wp_reset_postdata(); ?>
   	<?php else : ?>
   		
   	<?php endif; ?>
   </channel>
   </rss>
   

3. Basically you are done and you can go to your WordPress site http://yourblog.com?feed=custom. You may need add the following in order to flush your RSS Feed Cache – it’s a headache to wait for an hour to get your RSS Feed updated – change it to a second!

   /**
    * Setting a new cache time for feeds in WordPress
    */
   function prefix_set_feed_cache_time( $seconds ) {
   	return 1;
   }
   add_filter( 'wp_feed_cache_transient_lifetime' , 'prefix_set_feed_cache_time' );
   

P/S: You can customize your output by adding a bit of styling if you wanted to!