First off, Flat-spin, Rodeo, Underflip, Who cares? Just pick the one that seems most natural and try it out. Also, people have the names all mixed up and no matter what I call them, someone is going to say it's the wrong name. It could be debated all day long, but it's not worth my time. That being said, here's how I do a couple of these. If you have a trampoline or diving board, it is best to learn it there first.
Underflip (similar to a lincoln 180)- As you approach the lip, pop off your right foot while simultaneously throwing an uppercut with your right arm and reaching for a safety grab on your left ski. If you hold that position while looking over your left shoulder, the trick naturally completes itself. Just spot your landing and open up to stop your rotation.
Flatspin - I like to lean over on the right edges of both my skis while throwing my left elbow straight behind me and looking over my shoulder. Sometimes I'll carve up the jump to the left and then transition to my right edges right before I take off because it helps set my rotation. I also try to sit a little in the back seat to keep me from going bio. Once you're in the air, tuck it up and hold that position till you are back on axis. Flat 5's are the most natural for me.
Rodeo - Pop similar to a backflip but think about throwing your left elbow behind you and look over your shoulder. (It's almost like a chicken-out backflip) I also like to pop it a little to the right to take it a little more around the corner instead of just doing a backflip 180. This trick is somewhat similar to a flatspin but more inverted.
Anyway, hope that helps. Poppy jumps are easier to learn all three of these on. Good Luck. Late.