Stalker is a gear perk that greatly cuts your movement noise while crouched. It is a core stealth perk that lets you sneak right up behind guards for takedowns.
Crouch-walking makes far less noise. Guards are much less likely to hear you approaching from behind.
Wear the MK VII Tac Pants or Tactical Slacks in the legs slot. Gear perks are fixed to the piece of clothing that carries them, so you get this one by wearing that piece, not by unlocking it in a menu.
Travel Light and Silent Assassin round out a quiet build. Quiet Confidence helps you chain takedowns.