First you will have to spend a skill point in the skill tree and unlock "Weakpoints - Level 1" under the Skills tab, in doing this you'll gain the ability to hit weakpoints for extra damage, indicated by red circles on certain sections of the enemy. These can be revealed by parrying an attack. To deal weakpoint damage, move your target reticle over the red circle and use any attack. When done correctly the red circle will turn gold. I honestly got mine naturally after putting skill points into the "Weakpoint" skill.
You can track this under the "Merits" tab under the "Duelist" merit in the combat section.