Attempt to access Sequence Nine for the first time
If you are playing Assassin's Creed: Brotherhood for the first time, Memory 9 is the memory you open into. The only playable bit will be a quick time event where you attempt to kill Cesare. Hit X a couple of times until you lose control, and you should get the achievement during or after the cinematic.
If you somehow missed this achievement, start a new game. Don't have to erase your old file, just select from one of the two other save spaces. The part of the story this achievement goes off during can not be repeated, but you shouldn't be able to skip it so there shouldn't be any problem.