I will underline the sentences which are tips so it's easier to read the guide. If you know the basics you can just read these underlined sentences.

-You start the game at a train station full of zombies. They spawn from generators which you need to destroy. In later levels there will be many generators so try to destroy as many as you can fast before a ♥♥♥♥♥♥♥ of zombies spawn. Generators and zombies will also spawn on the lower floor which you can access via stairs. Once generators start spawning on the lower floor it's best to destroy the ones that are closer to you on the starting floor (leave at least 1 generator) and proceed to the ones on the lower floor.
-There are 4 types of weapons (pistol, shotgun, assault rifle, machine gun). Once you unlock a new weapon (explained below) it's best to play with it and only it especially when you have the machine gun.
-Red bar is your health. Try to have at least 60-70% health. In some situations that won't be possible but if you had higher health before you jumped into a horde of zombies or got surrounded by them you have a bigger chance to survive. Green bar is your ammo. In some situations it's better to reload even if you have few bullets left. However don't just reload after each kill.
-After you destroy all generators a boss will spawn (he never spawns on the lower floor). He is invulnerable until you have destroyed all his generators (they also never spawn on the lower floor). After that you can kill him.
-If you lose some health and kill a zombie you will drain it's health (or part of it at least). You might want to do that before a boss fight or simply when you have lost a bit of your health.
-When you start the game you will notice there is a bonus mission. Complete it and you will be rewarded. Complete as many of them as you can before proceeding to the next level. When you complete bonus missions you will receive stuff like new weapons, more health drain, faster movement speed.
-Ammo is unlimited (or at least it seemed like it is for me).
-Generators increase by 1 each level until they are 9 which is the max amount. After that zombies just become a bit harder to kill.
-You need to complete all 12 levels in one go. People in comments say that you don't need to complete all 12. You can complete 10, then first 2 or the first 2 then 1-10 (you get the idea) . If you die you start from level 1. If you follow my tips and you're decent at FPS games you should have no problem completing all levels.

Floor one looks like this (I ordered which generators you should destroy first):

Floor two looks like this (doesn't matter in what order you destroy the generators on this floor as long as you don't start the level with these ones and you have left at least one on the floor above):

Special thanks to my motivation and inspiration - this person

Thanks for being an awesome friend! <3