First you want to be sure that your skis are locked onto the rail, I had a problem before where I wanted to do a FS switchup without being completly locked on and they looked sloppy.
Once your skis are locked onto the rail, you want to do the scisor with your skis to try to push you to turn on the rail (press the heel of your downhill ski and the toes of your uphill ski). Your shoulders should always be 90* with the rail, don't turn them too early or you might get off the rail early.
When you start feeling that you are turning on the rail you want to pop somewhat high enough to bring your knees up and twist your hips 180*. When you land, you want to keep your head looking at the end of the rail but your whole body doing a 90* with the rail.
Basicly, always keep your shoulders 90* with the rail, don't turn them too early, do the scisor with your skis that's what's going to help you turn and pop on the rail and bring your knees up. Don't try to exit the rail facing down hill just yet, you'll pretty much always end up skiing switch after. If you feel comfortable, you can try to add a blind 270 out after if you do it on a box first. They come down pretty easily.
Hope you land it soon!