for FS go with your dominant side forward then dig your front ski edge in to the rail by angling your foot back then when you feel it catch pop a little and turn your shoulders everything else will follow.
For BS, depending on how many/ what kind you wanna do there's a couple ways.
1st on like a shorter box or if you're gonna do a bunch of switchups just jump on 90 and keep turning your shoulders the same way. It helps if you're jumping on the way you spin naturally. so if you spin left jump on right foot forward.
2nd is a little harder to get at first but is probably a lot more useful in the long run. for it you get on the rail and lock on. (you'll need a longer rail/box whatever.) then when you're ready for the switch dig in the toe of your forward ski until it catches. The longer you hold a pretzel, the more spin you'll get. Learning this kind will also help you a lot when trying to do BS spins off rails.
*You can also do a swivel, like on a snowboard. For those you don't pop, just turn your shoulders then quickly bring your skis around, kindof like doing a shifty 1. Those are super sick once you get them down, but not so easy to lear.
The most important thing to remember is to always be confident, look at the end of the rail and keep your knees bent.
Good Luck man switchups are super fun.
