Easy "Child Of The Shadows" achievement
The best place to complete a chapter without alerting anyone is Chapter one: Lockdown. This chapter is very short and has few enemies. The eye-symbol above an enemy's head must not turn fully red. If it does, they enter combat mode and you are then spotted. If this happens, immediately reload the last saved game. Always stay in the dark, do not shoot any arrows, and do not knock out any enemies. Stay crouched at all times, and use the Shadow Dash to get from one dark area to another when enemies are not looking towards you. Explore the area, and take alternative routes where you do not have to deal with any guards.