This is a step by step guide for creating a mega menu.
You can see the example of the mega menu from the :
1. Login to the Wordpress dashboard.
2. From the left-hand sidebar of the Dashboard, navigate to Appearance >> Menus
3. Select the menu which you want.
4. Click on a first-level menu item and check the mega menu option
5. Now drag&drop menu items under the first-level menu item, give them an indent, so they become the sub-menu of first-level menu item.