Briefings are build in HTML code, so you need to know that code.
stuff like <br> and so on.
Create an HTML file, call it Briefing, and basically that was it (not really).
But then the hard part begins, i dont (cant) stay here explaining you all the html functions and how they work in the briefings, best thing would be if you'd take a look inside at an already existing briefing file (and maybe play around with it modifiyng it), thats the best way to learn.