From what I've seen and attempted (water, not snow), flats are set almost the exact way as rodeo's are. The flat rotation is accomplished by popping and setting your backwards rotation that normally adds the invert component to your rodeo into more of a side flip. Basically, it feels like your ditching all that 'rotate back' energy to the side. I you get it going right, about .005 sec after takeoff, your should have your skis at 90, body coming close to 90 as in a lincoln loop. It might be easier to imagine this position and just try to make your body look like that right after takeoff, the rest of the rotation will follow.