i don't know about you, but there's nowhere near me that has a foam pit or water ramp. closest i can get is doing it on skates. i think a skatepark near me has them their. i try single flips on skates to practice for skis.
Learn cork 7's on the trampoline. Gradually take it into a double bug roll by doing the cork 7 and then keeping your momentum going until you land on your back- so you'll have a 1 1/4 cork to your back. Keep doing those until you get comfortable with them. Next just have someone throw a pad when you try it. By doing the bug roll you'll get yourself ready to do a double- all you need to do is make sure you commit to it on the tramp. Once its on the tramp, just decide where you feel like taking it- whether thats to water or snow.