WooCommerce Custom Orderby

gistfile1.php

<?php
add_filter( 'woocommerce_get_catalog_ordering_args', 'custom_woocommerce_get_catalog_ordering_args' ); 
function custom_woocommerce_get_catalog_ordering_args( $args ) {
  if( isset ( $_SESSION['orderby'] ) ) {
  	switch ( $_SESSION['orderby'] ) {
			case 'sku_asc' :
				$args['orderby'] = 'meta_value';
				$args['order'] = 'asc';
				$args['meta_key'] = '_sku';
			break;
		}
	}
	return $args;
}
 
add_filter('woocommerce_catalog_orderby', 'custom_woocommerce_catalog_orderby');
function custom_woocommerce_catalog_orderby( $sortby ) {
	$sortby['sku_asc'] = 'SKU';
	return $sortby;
}
?>

alias: it goes in your themes functions.php. Google "theme functions.php" and you'll see walkthroughs of how to paste code.