Through you're butler if you have one. If not, I think there is a line in the lobby each morning to get reservations. IMO that's the best perk of having a butler, one place to go for reservations.
Only 2 locations require reservations, unless things have changed very recently - 1. She - only two seatings each night. 2. Grill-side Hibachi at Sutra - multiple seatings per night