When you start a new game, after choosing which difficulty to play on, you will be given an option to choose your background. For this achievement, you must finish the game with the Recruit background. Using the Recruit background gives a slight disadvantage compared to the other backgrounds, as you will not have any skill points automatically invested like other backgrounds. That means you will end the game with less skill points compared to other backgrounds.
For your first playthrough, it is highly recommend starting off with the Recruit background, for efficiency. Finish the game with the Recruit background unlocks the Veteran background for future playthroughs.