For this achievement, the game asks you to complete all the objectives for every mission on Operative difficulty.
Here's the good news - there are no added objectives on this difficulty whatsoever. In fact, if you start a new game and select Operative difficulty, just go through the game as you normally would, completing each level and the main, primary objective. It's really that simple.
Of course, if you want to earn this achievement and all the other difficulty achievements the hard way, set it to Classic 007 difficulty and play the game from start to finish, which is what I did to earn this and the other four difficulty achievements. However, I would only recommended this difficulty setting for veterans or for those who wish to be challenged straight from the get-go! Otherwise, make a new save file, start a new game, select Operative difficulty, then finish the game on that difficulty setting. It's not that hard!