In order to follow this guide, you need:
As first, visit the admin panel of your WordPress website or blog, usually something like www.yourwebsite.com/wp-admin or www.yourwebsite.com/blog/wp-admin .
On your admin panel, visit the Widgets section, available from here:
The widget section will look like this:
So, after choosing the section, you must drag the Text box from the left to the section on the right, as shown by the screenshot below:
The widget will then open this way:
And you must paste here the iubenda embedding code, this way:
And hit save.
Now on your website you should see something like this:
The following video summarizes everything.