Creating directory and file using native wordpress file system

I am creating directory and a css file using conventional php method. Here it is

$upload_dir = wp_upload_dir();
$dirpath = $upload_dir['basedir'] . '/dynamic/';
$filepath = $dirpath. 'dynamic.css';

if( !file_exists($filepath) ){
  mkdir($dirpath);
  $fh = fopen($filepath, "w");
  fclose($fh);
}

But I want to use native wp filesystem .I have tried

if(!$wp_filesystem->is_dir( $dirpath) 
 {
   $wp_filesystem->mkdir( $dirpath);
 }

Now How can I create the file dynamic.css?

This is pretty tough for me to through this.
But You may follow this .

1.First let us check for the folder if it exists we will not execute anymore.

    if ( file_exists(trailingslashit(WP_PLUGIN_DIR).'demo' ) ) {
    exit;
     }

2.Now we will get the credential

    if (false === ($creds = request_filesystem_credentials($url, $method, false,    false, $form_fields) ) ) {
    return true;
      }

3.If we do not have file write permission ask for it

    if ( ! WP_Filesystem($creds) ) {
    // our credentials were no good, ask the user for them again
    request_filesystem_credentials($url, $method, true, false, $form_fields);
    return true;
    }

4.Everythig is good now let us create directory

    global $wp_filesystem;
    $plugdir = $wp_filesystem->wp_plugins_dir() .'demo';

5.Now we will create php vars or include file

    $header = <<<END
    <?php
   /*
   Your data variable. You can use include or require for separate file
    */
    END;

6.Our last part

    $plugfile = trailingslashit($plugdir).'style'.'.css';

if ( ! $wp_filesystem->put_contents( $plugfile, $header, FS_CHMOD_FILE) ) {
    echo 'Failed';
    }
    return true;

You can wrap the whole block in a function and execute it with suitable hooks.
Hope it helps