First before you get on the trampoline to do your back tuck you need to learn how to set. Setting is the preparation for your back tuck or in other words how to tuck back into it.
Please watch this video to learn how to set into a back tuck on the ground.
The is is how to set onto a trampoline make sure to watch this video because setting on a trampoline is different than setting on the ground
As you can see I'm setting before I go into a back handspring you need to do the same in a back tuck.
You want to make sure that your hands touch the ground before you lift them up quickly because if you don't you could get injured by landing on your head.
If you have never done a back tuck before you really need an adult to spot you. It is important because you need to get the feel of going straight up and tucking upside down.
When you set before you do your back tuck don't bend your knees all the way because it will make you stop on the trampoline.