Well, I spent hours and hours practicing cross and header and tried to find out the best way of doing it. And I can tell: it was definitely worth, because now it became a very important weapon for my team. :happy:
First of all, I play on PS2, then I am not sure whether works on next-gen versions.
I would say that the success depends 60-65% on the cross. The most effective is when the player who crosses is in the direction to the forward.
1) I go to the side with my player (SS, FW, SB, whatever) and then I fool with the defender going from one direction to the other. If I do in the correct time, the defender(s) cannot tackle or take the ball and I have time to check the position.
2) Here when I changed the direction and at the same time, I observed the position of my forward. I waited until he's in the middle or a bit close to the second post.
3) When I think the position is OK, I have to change my move in diagonal (see picture). It must be done in the correct time, otherwise the defender will come and take the ball. When you have two or more players marking the ball, it is still possible, but it's harder (but feasible :happy

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4) If you have done correctly, you will have time enough to stop your player with R2 in order to make him "look" at the goal (and your forward).
5) Cross the ball (with "O") and "use your head" to score.
Now I understand how important is to have a player with good jump and head abilities.