Nave Menu Register

1. Register menu in function
    // Register navigation
    function register_theme_menu(){
        register_nav_menus(array( // Using array to specify more menus if needed
            'main_menu' => 'Main menu',
            'footer_menu' => 'Footer Menu'    
        ));
    }
    add_action('init', 'register_theme_menu');
    
2. Creat fallback menu in function 

    // Custom nav fallback function
    function theme_default_menu(){
        echo '<nav class="main-nav">';
        if(is_user_logged_in() ) {
            echo '<ul><li><a href="'. home_url() . '/wp-admin/nav-menus.php">Create a Menu</a></li></ul>';
        }
        else{
            echo '<ul><li><a href="'. home_url() . '/">Home</a></li></ul>';
        }
        echo '</nav>';
    }


3. Calling registered Menu
// Calling manu in html 
<?php wp_nav_menu(array(
    'theme_location' => 'main_menu',
    'menu_class' => 'main-nav-ul',
    'container_class' => 'main-nav',
    'fallback_cb' => 'theme_default_menu',
    ));
?>