Go to WordPress, and click ‘Appearance’, and then ‘Menus’.
First, you’ll have to add all the menu items to the menu. You can select from Pages, Links or Categories. Check the checkbox in front of the item you want to add, and click ‘Add to menu’.
Now, simply drag the menu item that you want to make your submenu-item to the right, below the main item on the menu. To do this, leftclick the item, hold, and drag it to the right until you see the indented rectangle with dashed lines appear, and release the left mouse button. Now, save the menu by clicking the blue ‘Save Menu’ button.
Note: you can also make another level; sub-submenu items, by adding another item and dragging it below and right of the submenu item.
To make sure your menu is active, click the ‘Manage Locations’ tab. Here, click the dropdown menu, and select the menu you added your menu items to. In my case this is called ‘Menu 1’. Click ‘Save Changes’ to save the menu.
Done. Your completed menu with subitems could look like this:
With the theme I’m using (Iconic one Pro), your menu-items appear as a dropdown menu. In other themes, your submenu item can appear in a secondary menubar below your main menu bar, when the main menu item is selected.