Functions to add a custom WP cron

WP-custom_cron.php

function your_function(){
    // **** Add your function here **** 
}
add_action( 'your_cron_action', 'your_function' );

// Init Cron Job
function init_custom_cron() {
    if( !wp_next_scheduled( 'your_cron_action' ) ){
        wp_schedule_event( time(), '20min', 'your_cron_action' );
    }
}
add_action('init', 'init_custom_cron');

// Add custom schedule for Cron Jobs
function custom_cron_schedules($schedules){
    if( !isset($schedules['20min']) ){
        $schedules['20min'] = array(
            'interval' => 20*60,
            'display' => __('Once every 20 minutes')
        );
    }
    return $schedules;
}
add_filter('cron_schedules','custom_cron_schedules');