Check the range first..
If the free kick is located very close to the goal (16m-18m), use lay off free-kick, because it's very hard to curl the ball down after kicking it over the wall, you can try to curl with pushing d-pad down for a very slow and controlled shot if there's a short guy on the wall.. but as I said, shoot with lay off free kick to the empty side. Because when you pass it to the kicker in a lay off free-kick, the goal keeper goes back to the center of the goal and you'll find it very easy to score to the corner..
If it's close to the goal (20-21-22m), fill the bar halfway, or very very small up the halfway and give curl by pushing d-pad down, that'll give you a very slow curved shot.. (and don't use a player whose shot power is more than 85 if you're going to score over the wall, otherwise try to shoot to the empty side, and with a good curl, you can score to the corner the wall is not on..this is for close and very close shots)
If it's close-midway(23-24m) fill the bar halfway and shoot with X giving curl with only pusing d-pad to the sides.. that'll give you a slow shot, but a faster shot than the shot you'll make by pushing d-pad down..
If it's midway(25-26-maybe27m)Fill the bar little more than the halfway and give curl only by pushing d-pad to the sides, no pushing down, no X etc..
If it's long-midway (28m..)Fill the bar little more than the halfway and shoot with triangle, giving curl by pushing d-pad to the sides..
If it's long ranged (30+m) fill the bar a little more than halfway (like %55-60) and shoot by pushing d-pad upwards. that'll give you the strongest shot. remember: the ball won't get curled, it'll only swirl to the side you try to curl..
and if you want to score like from 40m or smt, use lay off freekick.. and when you pass it to the shooter, the keeper will come up front and shoot by filling the meter and it'll go in flying above the keeper..
glad if I could help..
and please don't forget.. always use free training mode to master your set piece skills...