THIS IS FOR THE SWITCH VERSION OF P5R, BOTH HARDWARE AND EMULATION SUPPORT MODDING
Requirements
Either a Switch, or Yuzu and Ryujinx
Criware File System
Installing Mods
Installing mods is pretty easy, the process is very simple.
You need to download and extract Cri File System linked at the top of this Tutorial, and you should see this.

Open up CpkFileBuilder.exe
After opening it up, navigate to your mod folder in your file explorer. Then drag Patch1 and/or Base and EN folders into the "Files" sidebar on the far left side of CpkFileBuilder.
It should looks something like this if you did it correctly.

Once you've dragged all the folders in, click "Build CPK" at the top. And don't change anything other than the name (and output folder if you want). You should name it "PATCH1.CPK" in all caps.

Then click "Clean Build" and let it build.

Once it's built, you have to drop it in a specific folder. It varies from hardware and emulator.
On switch it's /atmosphere/contents/01005CA01580E000/romfs/CPK
On Yuzu and Ryujins it's mods/contents/01005CA01580E000/romfs/CPK

Simply drop your PATCH1.CPK into the cpk folder and your mods should work!
Making Mods
Making mods is relatively the same process. You have to dump your game, using either Yuzu or Ryujinx or homebrew (many guides exist on doing this).
Open up CriPackedFileMaker.exe
Drag ALL_USEU.CPK into it

Now click the icon in the top right corner

Choose where to dump your game

Then let it dump! It could take a very long time, the cpk is very compressed which is why it's so small.
For resources on tools to use for files and extra help you can always join the Persona Modding Discord