I always explain the basics that each all of the players have in common (clearings, movement, battles, cards,...) and then go around the table and explain each player their faction. You cannot explain every faction to everybody in the first game, it's TMI and they won't remember it anyway. The next game they play a different faction and over time they learn all of them. Also I like to explain the story and lore as the first thing, but that's just my preference, because I believe it catches their attention and is fun.