Start small... as in skiing accross the hill, jumping, 180, landing backwards, and then jumping back to forwards.
After you can 180 on flat ground, you can do it off lumps of snow. Start going off kickers and tables, just staying still in the air. Then try tucking your legs up, try crossing your skis in front of you. Try tucking your legs up, and grabbing a boot - safety grab. Try tucking and grabbing a ski below the boot and pushing the other leg down - a liu kang. Try crossing your skis and grabbing one of them in front of your boot (left hand to right ski or right hand to left ski) - a mute grab. Now try 180's off bigger jumps and keep working on tweaking your grabs.
After that, go for a 360, or try out the halfpipe. You should be pretty confident in the air by now. I'm at that stage right now - looking to nail a 360 on the next good jump I see.