Please follow these steps. After following these steps you will be able to make your mega menu.
Go to Appearence> Menu
-First, create a custom Menu Like this
-Click checked on Mega Menu and Enable Big Mega Menu
-Select the column width as your needs. We have set six-columns for demo purposes.
-Check How other columns set under the mega menu
-Checked to “Display the category thumbnail” if you want to show images like this.
Set you categories option like this, and set how many items you want to show under the categories menu
Add child categories menu under the top categories menu
Set your label color and background
You can set images by using custom HTML code
Please see how to add other subitems