How to Add a Custom Admin Menu for plugin

add_action('admin_menu', 'myplugin_menu_pages');

function myplugin_menu_pages() {
    // Add the top-level admin menu
    $page_title = 'My Plugin Settings';
    $menu_title = 'My Plugin';
    $capability = 'manage_options';
    $menu_slug = 'myplugin-settings';
    $function = 'myplugin_settings';
    add_menu_page($page_title, $menu_title, $capability, $menu_slug, $function);

    // Add submenu page with same slug as parent to ensure no duplicates
    $sub_menu_title = 'Settings';
    add_submenu_page($menu_slug, $page_title, $sub_menu_title, $capability, $menu_slug, $function);

    // Now add the submenu page for Help
    $submenu_page_title = 'My Plugin Help';
    $submenu_title = 'Help';
    $submenu_slug = 'myplugin-help';
    $submenu_function = 'myplugin_help';
    add_submenu_page($menu_slug, $submenu_page_title, $submenu_title, $capability, $submenu_slug, $submenu_function);
}

function myplugin_settings() {
    if (!current_user_can('manage_options')) {
        wp_die('You do not have sufficient permissions to access this page.');
    }

    // Render the HTML for the Settings page or include a file that does
}

function myplugin_help() {
    if (!current_user_can('manage_options')) {
        wp_die('You do not have sufficient permissions to access this page.');
    }

    // Render the HTML for the Help page or include a file that does
}

Provide a Shortcut to Your Settings Page with Plugin Action Links

add_filter('plugin_action_links', 'myplugin_plugin_action_links', 10, 2);

function myplugin_plugin_action_links($links, $file) {
    static $this_plugin;

    if (!$this_plugin) {
        $this_plugin = plugin_basename(__FILE__);
    }

    if ($file == $this_plugin) {
        // The "page" query string value must be equal to the slug
        // of the Settings admin page we defined earlier, which in
        // this case equals "myplugin-settings".
        $settings_link = '<a href="' . get_bloginfo('wpurl') . '/wp-admin/admin.php?page=myplugin-settings">Settings</a>';
        array_unshift($links, $settings_link);
    }

    return $links;
}