Have you practiced 360s on a trampoline or just jumping off the ground? I always tell my athletes to practice them in their running shoes, off the curb, the steps, the ground. The more times you have done them without skis the better the result when on skis.
Once you are ready here are some pointers/suggestions:
Pop first, spin second
one of the most common mistakes I see with the kids I coach are that they try to do both at the same time and end up catching an edge as they leave the lip which usually puts them horizontal...not a good way to land. Be patient.....pop, then spin.
Find a reference point and use it
When you are standing uphill look for a refeerence point past the jump, could be a tree, could be a buddy that you ask to ski down to the bottom of the lnading area. when you are coming in, spot the point, pop, and look around for the point again, leading with your head.
Look over your shoulder, not under it
Another mistake that is common is trying to go "tucked up" or "off axis" the first time by looking under your shoulder or dropping your shoulder on take off. This is fine if you are experienced at T-Set 360s, but on your first attempts it is better to go old school and use a T-Set, which means your body is pencil straight, and you are looking over your shoulder for your reference point, leading with your head and arm. Do not stop looking at 180, as soon as your head stops leading, so to will your body.
Reach out for your landing
Even the best skiers in the world reach out with both hands for a landing, reaching out for a 4 point landing will help keep you balanced and increase the stability of your landing.
Don't be afraid or embarassed to use something small
Slow and low the first couple of times, find a small jump and don't worry about taking enough air to clear the table and land on the down slope. Casing the flats off a jump that is small is not going to hurt you, your point is to mkae it around at the moment, you can increase air and speed once you get comfortable.
Commit to the full 360
If you are not committed to the full 360 it will not happen. I see this all the time with my athletes, they are going in for a 360 but they are thinking 180 or 270 should be close. NO, you have to go for the whole thing
Once learned, 360s are one of the most fun and effortless trick you can throw, but learning them isn't always effortless for everybody, put in the leg work and you should be fine.
I hope this helps, and I hope you stomp some soon