This achievement requires you to complete the first mission of the game without being spotted.
Taking your time with this mission is crucial for this achievement. In addition, you can utilize manual saves to help you. Once you clear a section or a spot that might be troubling you, quickly make a manual save. This way, you won't have to restart the entire mission or revert to back an autosave if you fail.
Tips if you are struggling:
- Make use of manual saves, aka saving after each section so you won't have to revert back to mission start or an autosave
- Avoid enemies when you can. However, if you kill/neutralize any enemy, make sure to hide the body
- Stick to corners, walls and darkness
- It is good to look through keyholes before using a door
- Always remain crouching as you will make less noise