Gainers

you gotta thrust and jump to where u wanna go

i almost killed myself doing one once

well I suck at them so I don't know that I should be giving advice, but I can do 'em on diving boards and I find that you have to set most of the spin by like throwing your arms up and and kinda thrusting like someone else said. just look back and hope for the best, after all it's just water... and concrete.

well if youre doing it on a diving board: jump out to the water as if youre going to land flat on your back, then throw your knees to your chest and tuck. you will come around.

there r 2 ways of doing them 1 is u run and take off on 1 foot either ur left or right and just like pull urslef backwards thats the wya how i first learned but the 2nd way is much better and easier just stomp really hard on ur diving board and bring ur knees up and lean back simple ill try n post a vid sometime

your setup sounds... safe. its going to be hard to get enough air from a little trampoline (thats what you said you used, right?). traditionally on a diving board, the forward momentum would throw you out. so if you can figure out how to do that on a trampoline safely then windmill your arms from your legs towards your bellybutton and up over and behind your head. follow your hands and look back as far as you can. either bring your knees to your chest or do the other stuff more quickly.
you really just have to go for it though. theres no way to just get good at doing the body motions if you dont trust yourself. you just have to balls up and go. typically people get freaked out and build up a wall right before they would actually do the flip so they back flop. it is glorious to watch, it sucks to be that kid. (i was!)