Follow the steps to Add Submenu to Joomla Menus

1. Login to Joomla Admin
2. Navigate to Menus -> menu_name
3. Hit ‘New’ icon to create New Submenu
4. Select Menu Item Type (eg Articles – >Article Layout)
5. Add Title of the Menu
6. Select the Menu from ‘Parent Item’ under which you want the sub menu to be displayed
7. ‘Select Article’ you want to associate menu with from Parameters (Basic) on Right hand side
8. Save.