After hours of struggle before getting the Elden Ring Reforged to work on the deck, I just thought I'd share my guide to help anyone who is having trouble as well.
The whole process can be done with just the Steam Deck, but it will be much easier if you have a keyboard and mouse to pair it with the deck.
Also, this is for the Steam version. I cannot help if you have any other version.
Now without further ado, here are the instructions.
On the deck:
Make a clean/ vanilla save for Elden Ring. The best way to do this is to re-install your game, unless you know what files to delete. Just make sure to actually launch and play the game after re-installing/ deleting the files to actually make it a clean/ vanilla save. Also, make a backup of this save folder just in case.
Enter desktop mode and download + extract Elden Ring Reforged (ERR) to a temporary folder ("Download" is good enough)
Open "Steam" in desktop mode and click "Add a non-steam game" option.
In the new browser, click "Browse" and navigate to the folder where you extracted the ERR and select the file "Reforged UI Setup (run before first-time play).lnk" (If the file doesn't show up, make sure you change the extension option at the bottom of the browser to "All files")
Once it is added on the left panel, click the cog icon on the game page and change the compatibility to "Proton Experimental".
Launch the setup and follow the installer (once the setup is complete you can remove it from steam).
In the ERR folder, go into compatibility/linux and copy the "chr" folder to the "modz" folder (overwrite if prompted).
Move the "dll" and "modz" folder from the ERR folder to the Elden Ring game directory (HOME/.local/share/STEAM/steamapps/common/ELDEN RING/Game).
In the modengine folder, copy the "modengine2" folder and "modengine2_launcher.exe" file to the Elden Ring game directory.
Download and extract the proton enabled ModEngine 2
Inside the ME2 folder, copy "config_eldenring.toml" and "launchmod_eldenring.bat" to the Elden Ring Game directory.
Add the "launchmod_eldenring.bat" in Steam using the "Add a non-steam game" option and rename it to Elden Ring Reforged and change the compatibility to "Proton Experimental".
Launch the game.
If all goes well, you'll be able to play ERR now. Happy hunting.
However, if you are one of those unlucky fellas like me who gets the "Failed to load save..." error in the title screen, despite having a clean/ vanilla save proceed to the next set of steps.
Open the Elden Ring Reforged compdata folder. (If you have added the "launchmod_eldenring.bat" to Steam using "Add a non-steam game" and launched it, a new folder in the compdata should have been created. Sort the folders by "date modified" and find the most recently modified folder)
If you have successfully found the compdata folder, copy the vanilla save to the Elden Ring Reforged save directory.