Follow the below steps to make to make mega menu:

Steps

1. Go to Appearance → Menus in your WordPress dashboard.
2. Create or edit your navigation menu.
3. Add a top-level menu item that will act as the Mega Menu trigger.
4. Place the menu items you want to appear in the Mega Menu underneath this parent item.
5. Click Screen Options (top-right) and enable CSS Classes.
6. Expand the parent menu item and add the following CSS class:mega-menu
7. Save the menu and assign it to your website’s primary navigation.

Your imported menu will now behave as a Mega Menu.

For more details, please refer to Elegant Themes’ official guide: