WooCommerce - Add custom fields to order emails

functions.php

/**
 * Add the field to order emails
 **/
add_filter('woocommerce_email_order_meta_keys', 'my_custom_checkout_field_order_meta_keys');
function my_custom_checkout_field_order_meta_keys( $keys ) {
    $keys[‘My Field 1′] = ‘_my_field_1;
    $keys[‘My Field 2′] = ‘_my_field_2′;
    return $keys;
}