Are you talking about single player or multiplayer? Singleplayer you basically just have to hit a direction+X after a kill and your character will auto kill another character that you're point out. It's relatively similar to the past few AC games. Once you get a kill going, you can start killing in kind of a "flow" pattern. Its kinda similar in idea to Arkham City's critical hit attack, where you had to tap X in a flowing sort of motion to get the crits going.
For MP, its basically just killing or stunning 3 people without dying or losing a contract I think.
drointhewind :Now before I start I am not a sexist, I own a wife