Need help customizing the default feeds

Hi all,
I know this topic has been discussed many times on the forums and other sites, and the Codex gives some information on it, but for the life of me I just can’t seem to get it to work. So, I’m hoping someone can give me explicit step-by-steps on how to customize the default RSS (and Atom) feeds for WordPress.

I’ve tried editing the files directly in wp-includes, and I’ve tried removing the action and setting a custom RSS template in my theme directory using my theme’s functions.pho file (based on my understanding of the Codex article). Still to no avail.

All I’m really trying to do at the moment is: replace the “Author” with a custom field value, because all content is published by a single admin but attributed to the creator. After that, I may try my hand at some additional customization.

So, if someone can please just humor me and give me explicit instructions, I would be greatly appreciative. Thanks.