1. Navigate to the WordPress plugin directory of your development installation.
2. Create a new directory called ch3-multi-level-menu.
3. Navigate to this directory and create a new text file called ch3-multi-level-menu.php.
4. Open the new file in a code editor and add an appropriate header at the top of the plugin file, naming the plugin Chapter 3 - Multi-level menu.
5. Add the following line of code to register a function that will be called when WordPress is preparing data to display the site's administration menu:

add_action( 'admin_menu', 'ch3mlm_admin_menu' );

6. Add the following code section to provide an implementation for the ch3mlm_admin_menu function:

function ch3mlm_admin_menu() {
    // Create top-level menu item
    add_menu_page( 'My Complex Plugin Configuration Page',
                   'My Complex Plugin', 'manage_options',
                   'ch3mlm-main-menu', 'ch3mlm_my_complex_main',
                   plugins_url( 'myplugin.png', __FILE__ ) );

    // Create a sub-menu under the top-level menu
    add_submenu_page( 'ch3mlm-main-menu',
                      'My Complex Menu Sub-Config Page', 
                      'Sub-Config Page',
                      'manage_options', 'ch3mlm-sub-menu',
                      'ch3mlm_my_complex_submenu' );
}

7. Save and close the plugin file.
8. Find and download a PNG format 24 x 24 pixel icon from a site, such as IconArchive (http://www.iconarchive.com), resize it to 20 x 20 pixels, and save it as myplugin.png in the plugin directory.
9. Navigate to the Plugins section of your site's administration area.
10. Activate your new plugin.

11. You will now see a new menu item in the administration menu.

12. Expand the top-level new menu item to see the submenu item.