Actually I set up my template much as described in the example given in the docs:
http://docs.phplist.com/TemplatesInfo
I use an external HTML editor to edit my "newsletter.html" file. As the top part and footer part already hold the fixed elements (identical to my template, but for the full img tag urls), I only need to edit the middle part which corresponds with the message content part.
After I'm satisfied with the results, I have my online version of the newsletter ready for upload to the webserver (including images)
I then just copy the middle part to the content tab of phplist (use the source button of FCKeditor) and that's it.