Click the heart with a + next to it next to the station name while it's playing. When it's a favorite it'll show as a blue heart and the + will go away.
Thanks! I was playing with it today while my wife was driving (her first time behind the wheel of a Tesla) and I couldn't figure it out. It was fun playing Delivery Specialist and explaining all the features to her. Unlike my first time, she didn't put it in Neutral while driving (thinking it was the turn signal) I'm preaching the to choir with you but MAN I love this car.
Yesterday I did the opposite when I drove the ICE due to the S being clean and it raining. Went to switch gears from reverse to forward and turned on the wipers instead. Worst thing like that I've done is bright light flash people in the S when meaning to set cruise. Hate doing that. Feel like a jerk. Thankfully I think I've finally got the motor memory set up for that now though.
I find Slacker takes a day to update the favorite changes that you have made....you can also get your password from tesla and log in on a PC to delete favorites. Can't re-order though, they just appear in the order you added them.