To unlock the "Shhhh" trophy in HITMAN 2 you simply need to take down anyone on the level with a non-lethal attack. This will almost certainly be among the first trophies you unlock as pacification is key to high ratings and remaining unseen, plus the game's tutorial-like Prologue will give you the opportunity to learn Agent 47's full skill-set, including many options for pacification.
If you grab an NPC or knock them down, just make sure you choose options to knock them out rather than kill them. For example if you silently grab an enemy from behind, make sure to only tap
to subdue rather than
to snap their neck.