Definitely auto explanatory: complete a mission without causing (directly or indirectly) any death (civilian or guards).
If you're familiar with this franchise, you already know what to do. Otherwise you simply have to choose all the not lethal type of attacks (use
from behind and enemy, use electric bolt, use hooked mine to stun).
As soon as you complete a mission (1st one is easy enough) the achievement will pop.
To check your stats, hit
and scroll down to statistics, here you'll find how many times you've been spotted and how many people found death because of you. Simply check the value remains to zero :D
EDIT
Thanks for poiting this out: you can freely take down the dogs.