How to create classic menu locations on WordPress

register_nav_menus.php

<?php
/**
 * Register menu locations - Put this on your (child-)theme functions.php file or use a code snippets plugin
 */
add_action( 'after_setup_theme', function() {
	register_nav_menus(
		array(
			// One line per location
			'top_menu'    => 'Top menu',
			'footer_menu' => 'Footer menu',
			'legal_menu'  => 'Legal menu',
		)
	);
 } );