Extension manager>Installation:

• Install plugin
• Enable plugin
• The system plugin qldropdown adds javascript to your website.

Extensions>Modules>your menu

• Generate new Joomla! standard module menu (or use already existing one)
• Basic params
  • Set the menu settings to "display subitems".
• Advanced paranms
  • Menu class suffix: " qldropdown" for turning the menu into a dropdown; mark the _blank in front of dropdown!
  • Further Menu class suffix: " horizontal" OR " vertical" for horizontal / vertical positioning