Actually, its even easier than that
Go to
Editors Depot - Tutorials - Getting Started and download the
Tutorial Mission. It has all the extra files you need, like briefing, overview etc. Just copy them and insert your own text.
There are also tutorials on briefing and overviews in Getting started.
The best way to edit the briefing.htm file is to use a text editor like Notepad or Wordpad. Don't use html programme like Explorer, it makes things harder in the long run. Make sure you save the file as briefing.htm and not briefing.htm.txt