Start with skiing every run outside of the park switch.
You know how when you ski forwards and you carve your outside ski goes forward and your inside kinda stays back? Do the same switch. Look over your shoulder of the direction you are carving and keep your outside ski further downhill than your inside. When you get to the end of your carve even your feet out, look over the other shoulder, and switch your feet the other way. You will now be carving backwards. For hitting jumps or rails you'll want to keep looking over the same shoulder but you can still carve by changing the positioning of your feet. Just practice ALLLLLL the time and it will come. Skiing backwards is cool.