The first thing you have to do is to be able to trust your self going in a backward motion called a back bounce. The trampoline will be there, so you won't get hurt. This motion helps a ton.
1st Step: Trusting the trampoline
After practicing a back bounce for a couple times, and becoming comfortable with it, you want to stand up from it by flipping over. All you have to do is jump, do a back bounce, then flip over and stand up. Do this until you are really comfortable about it.
2nd step - Flipping out of the back bounce
After getting the feeling of leaning back, you're ready to do a backflip! All you have to do is jump really high, then lean back just like you do in a back bounce, but when you flip your legs over, put them straight down and land it! It is okay if you don't land it the first time. It might take a while but all you have to do is just jump really high and put your feet down.
3rd step - BACKFLIP!
After you master the backflip jumping, try doing a standing backflip without jumping. Make sure you use your arms a lot so you can get really high in the air.