Really fun if you know what you're doing. Tips down below in case you are having a hard time. Warning: MANY SPOILERS BEYOND NUMBER ONE
1: Difficulty
Obviously, you should start playing on Easy difficulty (aka Assisted) or you're going to have a really hard time.
2: Enemies
Zombies are easy to kill, but don't waste all of your ammo and just shoot to stun them. They might just be playing dead after you shoot them to the ground, so knife them as soon as you "kill" them. Lickers are a hassle to kill because of their high damage and speed. Mr X is like a giant, slow, mega Licker (he doesn't look like one tho). He is quite annoying, as he basically camps the area you are in. He is attracted to sound, such as running, gun-shots, moving stuff, and worst of all, CLOSING DOORS. It's basically impossible to not close a door because once you're far enough, it will automatically close and you will hear giant clobber stompers coming for you.
3: Ammo
DON'T WASTE AMMO. Ammo is surprisingly rare to come by. Don't shoot to kill zombies. DO shoot Lickers, as they are a huge nuisance if you don't kill them fast enough. Don't shoot Mr. X, as he is invincible to all damage at most parts of the story. He does get stunned after enough damage is done, but it's not worth it. Just run from him.
4: Healing Items
Herbs can provide many uses, such as healing or curing poison. The Red Herb can boost these effects. First Aid Sprays are much more rare, but heal you to full health. ONLY SAVE HEALING ITEMS when you are in the Danger zone (the lowest health in the game). Don't do it when you are in Caution or higher. However, there may be a few times where you are stuck in a cluster of zombies and you need to heal every chance you get. However, this is rare because most players wouldn't go flying into a zombie feast.
5: Mods for Weapons (Leon)
Matilda Extended Mag: in West Office, locked in Leon's desk, code left lock NED, code right lock MRG. Extends mag to 24.
Matilda Muzzle Brake: Locked in a safe behind the desk in the Waiting Room, code left 6, right 2, left 11. Decreases recoil
Matilda Stock: Inside of a police car in the Parking Garage. The key is in a box in the Firing Range. Once close enough to the car, examine the key and press the button. This will unlock the trunk, where the stock is located. Changes fire rate to 3-burst.
W-870 Long Barrel: Inside Kendo Gun Shop, on a shelf when you come in. Increases capacity and damage.
W-870 Stock: Found in a safe in the Treatment Pool Room in the Sewers. Code left 2, right 12, left 8. Increases fire rate of the W-870.
Lightning Hawk Long Barrel: Locked in a case under the police station. Change the USB Dongle Key back into the RPD badge by examining it, then place it in the slot to open the case. Reduces recoil and increases damage.
Lightning Hawk Red Dot Sight: In the drawer in the room of the S.T.A.R.S office. Enables lightning-fast aiming.
Chemical Flamethrower Regulator: In a locker, in the Nap Room of NEST. Increases fuel life.
6: Mods for Weapons (Claire)
SLS-60 Speed Loader: In Leon's desk. Greatly decreases reload time.
SLS-60 Reinforced Frame: Found in a safe in the Treatment Pool Room. Allows use for High-Powered Rounds.
GM-79 Stock: In the Elevators Control Room in the parking garage. Makes shots more accurate.
JMB HP3 Extended Mag: In the safe in the Waiting Room. Extends mag to 26.
MQ-11 Suppressor: Found in the RPD Badge glass case. Silences weapon.
MQ-11 Extended Mag: Found in the drawer of the S.T.A.R.S office. Extends mag size to 50.
Spark Shot High Voltage Condenser: Inside the locker of the Nap Room in NEST. Increased fire rate.