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

Just look at the main menu above, well, yeah, you're right; that's the qldropdown.

Mark the css classes added to the menu class, which are "qldropdown jquery".

The system plugin qldropdown adds javascript to your website.

Any menu with the classes "dropdown js" is turned into a dropdown menu vis javascript.

Any menu with the classes "dropdown css" is turned into a dropdown menu via css.

Mind to set the menu settings to "display subitems".