ok, well, this trick is quite an advanced and technical aerial manoever, so you will want to practice going off jumps and doing nothing in the air a few times before you attempt an iron cross. make sure you practice on the jumps with the biggest lineups of the best skiers, as you will learn from them and when you accomplish the iron cross manoever you will become one of them.
so, you want to have decent speed approaching the jump, but if your scared, dont go too fast. landing on the deck (skier-talk for the flat part of the jump before the landing) is absolutely fine. when you take off, you may want to "preload" your manoever, i.e. cross your skis slightly before you become airborne. when you are in the air, pretend you simultaneously get sack-tapped and have a long hard object rammed up you anus, but be happy about it. you can show this by punching your fists out, but DO NOT SMILE. make sure to bring your skis back to the french fry position before you land, as to avoind crashing. when you land, ski away and have no reaction. you are now a freestyle race expert