I saw several other Guides on this topic already, but they seem to be outdated and state things that are no longer necessary. So here is an updated guide that makes setting up a PS2 Core in RetroArch the easy way, as well as steps to get the BIOS File and ISO.

• RetroArch on Steam
  
• Internet
  
• USB Drive
  
• Modded PS2
  
• Disc Drive
  
• a program to unzip files
  
• a program to convert discs to ISOs

You no longer need to download the standalone versions of either PCSX2 or RetroArch to do this, all you need is the steam version of RetroArch and the files listed below.

• First, download the current, up to date RetroArch cores from their buildbot[buildbot.libretro.com]. Just to reiterate, RetroArch itself is NOT needed. You already have the Steam version, and that is enough.
  
• 
  
• Next, download the up to date info files from the same website. You will need both of these for the core to work.
  
• 

After obtaining both of these, extract the folders using a program like 7-Zip and move on to the next section

Next thing you'll want to do is move both of the files to their respective locations within your retroarch directory. If you do not know how to find that, simply hit the "Browse Local Files" button on steam

• Move the dll file for PCSX2 to the Retroarch/Cores folder like so
  
  
• Move the info file to the Retroarch/info folder like so
  

After that, we are all done and the Cores are almost ready to use :D. You don't need all the other files, they are for the other cores. We don't need those for this, although they are there if you want them.

If you already have your BIOS file, feel free to skip ahead, if not, then follow along. This will likely be the most time-consuming part of the tutorial. First thing you will need to do is find a program to take the BIOS file from your PS2. I used a program called BIOSDRAIN[github.com] for this. The program is Biosdrain dot ELF.

• First up, Load Biosdrain dot elf or whatever other program you are using onto your handy dandy USB drive
  
  
• Shove it into your Modded PS2. If your PS2 is not modded with either FreeMCBoot or FreeHDBoot this will not work. I highly recommend modding your PS2 anyways as it allows you a wide degree of freedom on what you can do with it.
  
  
• Select ULaunchELF to bring up the file browser
  
  
• Select mass for mass storage (your USB) and run BIOSDRAIN
  

All we can do now is wait. It can take a minute or two, but not too long. The little bongocat character will entertain you in the meantime until BIOSDRAIN says it is all done. From here just shut off the PS2 and take the USB drive out.

Just like with the BIOS Files, if you already have your ISO for whatever game you want to play, skip ahead. For people who do not have access to any ISO files, this one is for you.
For this Tutorial I will be using Raidou Kuzunoha Versus the Soulless Army. It just had a remaster get announced and maybe you'd want to play the original with all the graphics and gameplay untouched, or maybe you'd just like to see it in higher resolution.

• Either way, first thing's first, you'll need to grab the game
  
  
• Shove that bad boy into your Disc drive on your PC
  
  
• Load up a program like ImgBurn to turn the disc into an ISO, as currently the emulators don't run programs directly off the disc that well. Make sure you place the ISO somewhere you can find it.
  
  
• Let the program work its magic, this may take a while, especially for larger games like some RPGs
  
• when you get the notification that the file is complete, hooray, we are done with this step
  

• Launch RetroArch on steam and verify that the PS2 core is there. The name is LRPS2. Select it and just do a basic check to make sure everything looks normal (except the missing BIOS and Games!)
  
  
• Go to the System folder for RetroArch, create a new folder called PCSX2. From there, create another new folder called BIOS
  
  
• take the BIOS folders from wherever you have them stored (in my case, my flash drive), and
  
  
• The BIOS are now in the folder EXACTLY where they need to be
  

Your version of RetroArch on steam is now good to go :D I recommend putting all of your ISO files somewhere easy to access and remember. Feel free to play around with your settings to see what works on your PC and what makes the game look good for you. I know for me, I need to use Vulkan and disabling V-Sync helps sometimes.

You can now play these games with Higher resolutions, Widescreen, savestates and more. Have fun.Original Resolution
Widescreen and 1080p