Editing vanilla 'use' of items and usage of the 'ItemCheck()'-related methods

Hello and good day,
I am willing to perform changes of my own to the game and as the title reads, one of the things I want to change is the usage of certain (vanilla) items and their effects.

First of all, I have gone through the game's code and found where are these "usage effects" are defined and executed(ItemCheck() method in Player's class file), but I haven't been able to understand how to approach changing them. I have also tried searching for guides on how to do so, but...

Editing vanilla 'use' of items and usage of the 'ItemCheck()'-related methods