down rails are hard to master at first. there are 2 methods:
Popping on- exactly sounds, come from either side of the rails at a bit of an angle, pop hard and center your feet on the rail. Don'tcome at too much of an angle though, as this will cause you to fall right off the other side.
Stepping on- this usually used for higher down rails, but can be used for both. You need to step off your outside foot and put your inside ski on the rail sideways. Then take your outside ski, place it on the rail and fully commit to it. This will take time and practice to get used to.
in my opinion, popping on is easier and should be done first. Both ways are going to take time to get used to though, hope i helped!