I checked the help options on ps4, but couldn't find it.

You have to have the perk, pull your gun out and aim at a lower level enemy.

But which button do I press when I Aim at the creature?

you just aim at it and it should work

You don't press any buttons. Just point gun at the enemy and aim.

you shoot the enemy in the head repeatedly until the stops moving - then hes pacified

If you are on PS4 its X. Make sure the enemy is in your crosshairs and it should say "Pacify X"