Source for the installation script
Download and use the script :
# when asked, install to directory with ffxiv_dx11.exe (Game Path/game in xivlauncher settings)
curl -LO https://github.com/kevinlekiller/reshade-steam-proton/raw/main/reshade-linux.sh && chmod +x reshade-linux.sh && RESHADE_ADDON_SUPPORT=1 ./reshade-linux.sh