This one is pretty straightforward, simply sneak up on an assassin and assassinate them. Make sure you attack them from behind and get the stealth kill animation as well.
You can do this as early as mission 3 (The House of Pleasure), as there are a few Assassin's stationed outside of Slackjaw's distillery, however, there are way more opportunities in mission 7 (The Flooded District) since the assassins are everywhere