You'll get it playing normally: the assassinations are, as well know in this franchise, when your hero kill some enemy while unseen.
With a stealth strategy you'll unlock it during Sequence 3 or 4, but you can speed it up killing every bad guys you'll meet on your path (the guys glowing in red): since they won't attack you immediately, you have the chance to reach them, kill them hittinh

and continue on your way without any trouble.