Cover Art (again)

Firstly let me apologise for asking a question for which there are already plenty of answers to be found using "search".

I have thousands of M4A files all nicely tagged the way I want them....using MP3tag of course.
I now want to add cover art without retagging and have found the required scripts. Unfortunately I don't understand scripts and how to use them as I am a 60+ years boring old fart.........!

Can someone produce an idiots guide for this?

Thanks

What scripts do you mean? Do you want covers from web sites?
Then read freedb and Web Sources