Navigate to Development - Menu, open the menu list page. 

Create a menu

To create a menu, click the "Create" button

After a menu is created, click the menu name, open menu editing screen. 

In above screen, 

Click "+" icon to create add a sub menu item.
Click "-" icon to remove a sub menu item. 
Click "</>" icon to open the menu template editor. 
Click on the text or link to edit text or link. 

Edit menu template

Click "</>" icon in above screen, open the menu template editor. 

Menu template contains Html code and merge fields that will be replaced by menu data. 

Edit the "Sub item container" and "Sub item template".

Click the text button(e.g. markSubItems), will insert the related merge field into template. 

The Preview panel will display a sample result of your menu template with sample menu data.