You need to know how to edit the templates, which are written in the smarty language. I don't know if plog has any documentation about that,
http://smarty.php.net explains things, although it is a lot of stuff, I found it easier to just read through the templates and try adding text where I wanted to, and eventually get tricky enough to replace stuff within the { }.
As a plain user, you don't have permission to edit individual blogs (at least the way I set it up for someone on my server, perhaps if you had 777 permission on your template directory you would, but the way I did it, was copied a template to a blog specific template (using the template editor, there must be a copy button or something) and then the user can click on the template name, and then see all the files, and open and edit the files.
There are two places to look at template editor stuff, if I am remembering correctly - inside the control center, and inside the administration section.