Early in the first level, you will come to a part where you will have to sneak past a guard. Once you hide in the dark entryway with
, the guard will turn around and head to the direction you came from. After he's past where you are hiding, you can leave with
and continue to the next area.
Given that you don't alert him (It's really hard too), you will get a Shadow: Gold rating.
Once you see that pop up, pause and hit
restart checkpoint immediately and you'll start behind the guard again.
You can get the Shadow: Gold rating through normal gameplay by remaining unseen and not killing enemies
except for those that the game makes you kill.