So ...

Before anything else, I highly recommend you to create a full website backup before making any changes!

Also, if you click the direct file name link ...

... you'll be taken to the default Wordpress theme file editor where you can - directly - edit that particular file:

Don't do that! Many things could go wrong and there is no undo option here!

ALWAYS use an FTP connection or a file Manager plugin to download, edit an re-upload the targeted template file. This way you'll be able to create an offline backup copy of your original template file.

And one more thing ... Don't forget that the new "What The File" menu item will be visible to all logged in users! So be sure that you have clearly defined user rights for your authors, guest bloggers or anybody else who can access to your admin area.

And that's it my friends!

If you have any comments, further questions or update requests please don't hesitate to react! Like, comment and share!

And don't forget, you can easily come back to this tutorial whenever you want.

All you need to do is to bookmark the categorized, daily updated register of my blogs and training materials. Here it is:

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Thanks for your time!