She's in the Vents... (20)

Use the vent system 20 times

This achievement should unlock naturally as you continue to play the game.

If you want to quickly get it out of the way, you can do this achievement near the start of the second mission "Welcome to Sevastopol." You will have to enter a vent to proceed in the game.

Enter the vent with A , then move the left analog stick down so you are crouch-walking backwards out of the vent. Stop moving when you are prompted to enter the vent with the A button again, then reenter the vent.

Repeat until the achievement unlocks.