The overview is a HTML file like the briefing, but a bit easier to handle. A finished overview.html has been added in the Example Mission.
As you can see in Image 1, I've added an image to the overview. This is something highly recommended! Adding a title, (Example Mission) is something I appreciate too.
Don't make the text too long, try to keep the overview.html short to prevent it from being spread over two pages. A one page overview beats a two-page one.
I suggest using the template in the Example Mission for future missions. There is no point in making them yourself over and over again while you can just use an already existing one and then slightly adjust it where required.
Note that in multiplayer missions, you don't need an overview as it won't get displayed. To check if your overview 'works' you'll have to save the mission to your "missions" folder (an option after pressing "save"). Then go to the missions list and take a look at the overview.