New to FCKeditor - How do I create a new toolbar button?

I've searched for this answer with no luck, so I do apologize if this has been asked previously. I'm trying to add a button to the toolbar that will execute a simply javascript on the content of my text area. I can't seem to figure out how to add the button though. I don't need anything fancy, I just want to be able to create a button that I can click that will execute my script on the content in the editor. Any ideas? Thanks!