just relax and commit your whole upper body and start setting the spin before the end of the rail, by the end of the rail your shoulders should be facing up the hill.
my advice, learn blind switchups first, cuz that way it's like learning 2 tricks in one, because you can just throw the switchup later on the rail and you do a 270 out. blind switchups are way harder on rails, so learn them on a box and then just take the 270 onto a rail, but 270 outs on boxes without any tricks before them is both hard and gay