It's not hard at all. First, you have to backup all of your stuff on your phone (the easiest way is through recovery mode, if you already have flashed a custom recovery).
Then you have to perform a factory reset (this will delete EVERYTHING, so make sure you made a backup before this step). You also have to do this through recovery (in CWM recovery, there is an option available. In TWRP, you'll have to select wipe and there you can do a factory reset).
And finally, you can flash the zip file by selecting 'install' through recovery and selecting the zip file you want to flash.
If you have any more questions, feel free to ask them
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