Get control on small jumps, you have to spin fast and have a good pop, make sure your whole body spins at the same time
Then try it on a low flat box, make a good pop and make sure to land with feets wide appart, try to enter the box from the side, not with the box between your legs
To get used to the feeling of a rail the best alternative is Pvc pipe, set it lower than the kicker at first but put it higher as you get better, this will practive you to get the tips of your skis over the rail before spining
once your good with this you can try to do down rails and boxes
Once you get it right it should be easy every time, muscle memory does the work