Keep hidden as you proceed to avoid being found by guards, cameras, and sensors.
When you receive a codec call from someone, you'll see the CALL sign. Try to accept these calls as much as you can, as they could provide important advice.
If you use a firearm, the enemy will hear the gunshot/explosion and discover you. When using a handgun or submachine gun, use a suppressor to keep the enemy from hearing gunshots.
Try to keep your ration and ammunition supplies as full as possible for the battles to come.