First, install the “OT Mega Menu” plugin for your theme.
After activating the plugin, you go to Appearance > Menu to setup.
At level 1 menu item, click the arrow on the right of the item to reveal additional configuration options. Click the “Settings” button to start the mega menu builder. ( Note: You can only build mega menu at level 1 menu item)
Select menu item to build mega menu
Select mega menu and width of mega menu
Note: Mega menu is only visible on desktop devices, not on mobile devices.