Let me explain this as someone who went into this game as a British person without knowing a thing about American Football, because whilst this achievement should have only taken me about 15 mins to do it ended up taking me about an hour to figure out how to do this desipite everyone's excellent descriptions, due to all the terminology used.
A sack is obtained when the person in the quarterback gets tackled whilst holding the ball and whilst also behind where the ball initially started in the middle of the round.
The quarterback is the player who stands behind the main defence row of players and is generally the first person the ball is passed to at the beginning of the match.
You can adjust all of the AI and Game sliders to the bottom, so that the referee is super lax about everything and so that you can literally run your controlled player over to the be literally right next to the opposing teams Quarterback. That way as soon as the round starts and the ball gets passed to him, you can just press X to tackle them straight away. Just make sure that when running over to him you do not touch any of the opposing teams players, and the ref will do you for that.
Do this four times (I don't think it has to be the same player, but I did it just in case) and you'll get the achievement. Very easy when you know how.