Flatspins on the tramp for me are either rodeos or bios, I can't really get between the two.
On skis, I learned backflips, then flat 3(wackflip), then started trying rodeo 5s which ended up looking really hucked because I would absorb the lip so much when I took off. I started to try popping off of the lip before I set my rotation which is what really helped me. Ski up to the jump with your knees bent, and sort of jump as you take off the lip, and spin or throw your shoulder forward horizontally(right shoulder if spinning left). The amount I pop and if I try to keep my head up during the trick are what differentiate a rodeo and a flatspin with me. Also, if I grab above the binding I always do a flatspin. Grabbing japan usually makes a rodeo, but I find japan the most fun. If you set the trick right and commit, you should bring round a flat or rodeo 5 just fine. Hope this helps