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 silencer to keep the enemy from hearing gunshots.
Try to keep your ration and ammunition supplies as full as possible for the battles to come.
Use continue to resume the game with all your weapons and items from the beginning, giving you the advantage in your mission.