A sack is it is when you tackle the Quarterback before he gets a chance to throw it. You can only sack the Quarterback if they have chosen a pass play, which means if they are giving to a Running Back and you tackle them it will not count. This can be quite tricky trying to do it legit against the CPU. Luckily, there are are easier ways to do it.
You can use the two controllers technique. Control the Quarterback with the second controller and run him into one of your players seven times. Or you can turn off the offsides in the Game Settings. To do this you have to be on the main menu. Go to "My Madden" then "Settings" and then "Gameplay". Press when you have the "Game Rules" highlighted to modify them. Move the offsides slider all the way down to zero. These methods can also be used for "8 Tackles with One Player".