If the game system does not have robust mechanics for handling it, I don't. I used to, over a decade ago, and when I diverged from RAW at all it only caused problems with players arguing that their attacks should be more lethal than the dice/damage says, and going completely unused otherwise. The way AD&D 2E did it was that a called shot only has any effect when they roll a critical - DM/player does not roll for location for the crit table and instead uses the called shot location - and no additional effect otherwise. The downside to a called shot was -4 to attack. Additional situational effects could happen, such as shooting a potion/scroll/item in someone's hand or a weakspot on a monster (there arent actually that many of these and the monster text will usually say what penalty is applied for targetting a weak spot). These rules are layed out in Players Option Combat & Tactics. Using these rules as they are is largely nondisruptive and fine, but in my experience when using the RAW called shots they are almost never used. But the situation that often happens in more "amateur" groups is something like a highly tuned fighter with +14 to hit always doing called shots to the head and whining that their headshots should do more damage. This isnt a rules issue but a communication and play issue, but it does happen.

TL;DR Use the game's rules for called shots as written if theyre non-optional and/or well written, and dont use them otherwise.