Crossing has always been one of the easiest ways to score in FIFA. Next to corners, it is the hardest thing to defend well in FIFA. If you do everything right, your opponent can be simply helpless in defending your crosses.
The first thing to master is the three different kinds of crosses. Most people think that you can only hold down the lob button to put in a lofted cross. However, you can also tap the cross button twice quickly for a driven cross and tap it three times for a grounded cross. These other two crosses are useful when you can cut inside and will have two or more players waiting in the penalty box for a cross. This way, you can drive the ball across the line and in front of the keeper. Hopefully, this will rocket to one of these players, but what makes this really useful is that if your initial cross is unsuccessful, you can count on the ball jumping around in the penalty box for second chances -- you will have other players running in to help you put it away!
The first objective here is to get your player as close to the goal-line as possible. If you’re out on a wing with a single defender tracking you down: ALWAYS GO FOR THIS. As soon as you get past him you’re usually free to run towards the goal-line and the goal itself to set yourself up for this kind of cross. The image below illustrates this approach.

As you can see, the Manchester City left defender has just been dribbled past by our crosser here. The player making a slide tackle illustrates how difficult it is to stop a winger who has crossed this estranged outside defender. He has had more than enough time to get to this position we’re talking about. From here, a driven or grounded cross would either result in a direct goal, a goal from scraps, or a serious chance. If you’re unfamiliar with this kind of tactic, try this out in the practice arena a few times to get the hang of it.

One last note about this kind of cross: The grounded cross tends to be directed very drastically forward. What I mean is that if you’re directly across from an open player, your crosser will sometimes completely miss him because he crosses it too far ahead of him. This is why it is important to get to the goal-line for this kind of cross. By that time, your target will likely be towards the goal-line as well and will simply tap it in from where he is standing. Just be wary of how off this kind of cross can be. It is often best just to stick to the double-tap cross unless you really know how both crosses work.

The traditional lofted cross is more based on luck. However, you can seriously increase your chances of scoring off the lofted cross by prioritizing your aiming. You have one thing you can really impact with this kind of cross, and that is power. When you have a crossing opportunity on the wings, you need to survey the penalty box to determine whether you should aim towards the center of the box, or towards the far post.

The highest conversion cross at this point is a longer, more powerful lofted cross to the far post IF you have a player making the far post run either alone, or ahead of his defender. FIFA AI is getting better -- this kind of run is made all the time, usually by opposing wingers.

As you can see in the image above, you can simply loft the cross above the two central defenders for the player running inside from the opposing wing to head in. If you get the power right (higher than a central cross but not too much higer) he should have an uncontested header towards the end of the penalty box. Often, if you have even more time to let the cross go down, he will have an opportunity to volley or even control for the finish.

A central cross is very difficult to pull off uncontested, unless you’re on a pretty serious counter attack. However, you can still have a pretty good chance of heading these in if you’ve got a strong central forward. This is why it really helps to plan what kind of cross you’re looking for depending on the types of players you have. If you don’t have that huge Adebayor or Balotelli type player, just plan ahead to go for the driven cross or to go for that lofted cross across the box as early as possible. If you do have a big central forward, go for it!

FIFA Version: