This is probably the most important one… SAVE! Even the game encourages you to do so. Save your game at every turn if you want/have to. There’s no harm is saving and you’ll regret it if you don’t.
Take your time. During your first playthrough, take your time. Every mission has a badge for speedrunning the mission, but you want to focus first on getting used to the game and learning the mechanics.
Hide. Use anything you’re given to hide. You can usually hide in bushes, but as the game prolongs you start finding hay bales and even wardrobes and whatnot. Do be careful because you cannot hide in houses or mine shafts. Even though you can go in, if an enemy goes in they’ll see you.
Hide the bodies. You’ll want to hide the bodies of all the enemies/civilians you kill/knock out. Besides helping you keep the place tidy and not risk getting caught, it helps you get close to a trophy that requires you to hide 750 bodies.
Don’t be afraid of using your firearms. I played through most of the game without using any firearms, which is kinda crazy if you think about it. Unless stated in the guide, I didn’t use a firearm at all. Don’t be like me and use them. There’s plenty of chests in each stage that replenishes your ammo supply.
Long Coats. You’re told about Long Coats in Mission 3 and these can only be killed, or one-hit, by Hector. Don’t depend solely on him. If you can shoot the enemy and stab him afterwards and nobody will hear you, do that instead of risking getting caught.
It’s not necessary to kill all the enemies in every mission. If you can get by just killing the ones you need to kill, then do just that. No need for you to spend extra time and effort killing them.
Learn the enemy movement. Before you enter an area, take a minute or two to watch the enemies and see how they move and what they do. There are enemies that stay still and just move their heads, while there are others that patrol an area, there are others that patrol an area PLUS they talk to another enemy, making them turn around and decrease their viewcone.