This is most easily done on Mission 4: Fort Rifugio
on Cadet difficulty while going for the "I Fort this would be difficult
" achievement. You must complete the entire mission without being spotted. Being spotted happens when an enemy gets suspicious of your location, sees you and then a full red circle appears above their head. To avoid being seen, make sure you are sneaking around quietly, and stay crouched whenever possible. Try to approach enemies from behind and take them out using the Welrod which is silenced. You can also try to hide the bodies, but that isn't necessary.
If at ANY POINT you are seen and an enemy has a full red circle above their head, just pause the game and select "Load Last Save" to try again. The achievement is unlocked upon completion of the level. It should also be noted that the player can go loud at the end of Mission 4 as the sirens will mask the sounds of your rifle.