So, the reason the first step is cardio because you have to be able to out-run a zombie in lot’s of situations. Good cardio means that you would be able to outrun and get away from zombies that are chasing you. That’s why good cardio is necessary to have because who wants to get caught being slow.
Trigger finger is important because most zombies will not die from just one hit unless it's a shot to the head, and it will instead get up in very little time - and will then bite
you. The most effective way to prevent this is to take a second shot/hit to their head which will stop them from attacking again, and surely one more shot cannot do too much harm. For this to work, you will need at least two shots in your gun at all times so you can double tap without having to worry about reloading in time. This is why you would need a trigger finger.
You will need to be careful when using the bathroom because this is the most vulnerable place. Zombies seem to sneak up on people who are at their most vulnerable, although it probably just seems like this as zombies just attack with no special circumstances. The best way to stay safe is to check each bathroom before you enter, in every cubicle and every section as it isn't rare to find one just waiting there.
All of these steps are important to follow if you want to survive. Just remember these and you will definitely have a chance at survival. Now, just remember it is important to do these steps cautious because you don’t want to snuck up on by any zombies.