It seems to me that this would depend greatly upon what you're trying to do.
Called shot to a weak point (eyes, head, etc.) seems like invoking an aspect that already exists, pay your fate point (or tag) and take your +2
Called shot to weaken the enemy (trips, attacking knees etc.) seem like maneuvers. You can then use your tag (or one of your own fate points) to compel them not to move, to miss, etc.
Additionally, you could (after working with your GM) roll your attack normally, see how you do and how much damage, and if there are any consequences. Based on what consequences are available and what you wanted to do, you can describe your actions and pick consequences based on your called shot.
I tend to prefer the last option, I decide a general goal, throw the dice, and then based on what is shown the GM or I describe precisely what happens. It makes for a much more cinematic style of combat.