What should I do to train and prepare for SFAS?

Leg day everyday

Learn not to be an asshole. You have to work well with others. Be the last one to sit down and the first to get up.
Physically, it's just not that tough. There are a handful of hard events, but there are a lot harder schools and selections. I felt that RIP, Ranger School, and even Jumpmaster were more difficult. It's the peer evaluations that will get you. So, work well with others, carry your weight, and be an asset to the team.

They are looking for a thinking mixture- no Arnold's, no quitters, no Rocks. Improve your body, do lots of rucking, study up on land Nav, field training, but it won't work less you can push yourself to beyond your limits.