1. WINS: You need a certain # of wins to attain Guards status. It's not automatic, but usually you need 9-ish or so. Once you hit that, unit will eventually convert to Guards status 2. You can combine Airborne Brigades to make a Guards Rifle Division. I wouldn't do a ton of that, as the airborne bdes are useful
That's it, most of the time it's #1, and that applies for all units