I'm just learning corks on the tramp but I'll tell you what I do to get them. (Also try the dropping your shoulder tip 1st, the only reason I am doing it this way is because that didn't work for me) First you should learn the back drop. For this the idea is to land flat on your back with your legs up and in a sitting position. (try and imagine that in your head) It will help later if you keep your arms crossed over your chest. To get like this you want to jump (low at 1st but get higher when you feel confident) then you should raise you up, this motion should make you lean back. Then just fall back flat to the trampoline bed.
The next step is to work on your spins, just straight up 1's 3's, and 5's. After you get comfortable with those work your way to 7's. Practice is key, I had a lot of trouble spinning on the trampoline.
The final step is to combine the back drop and spin. The problem I had with learning corks was that I would through my body back, almost of the tramp on not get the spin part. I thought that it would be a good idea to combine the movement of bringing your shins up for a seat drop and spinning at the same time. It worked well for me, the only problem is that you either land on your back or chest, I'm still working on getting back to my feet but its a start. Also start with smaller spins then work your way up.
***Sparknotes*** Lift your shins before you go to spin, that will get you vertical, then just spin (You will land on your chest or back depending on the rotation.
Trampoline cult if you need more info:
https://newschoolers.com/web/members/cultforum/cat_id/5155/
Good luck.