Put the iso file for Gihren's Greed AND the xdelta3.exe file in the same folder as the EBOOTPATCH file. Open power shell, and navigate to the folder containing all three files.

So, replace the bracketed names with your actual file names:
xdelta3.exe -d -s [Original_Game_Name].iso EBOOTPatch [New_Translated_Name].iso

-d is a flag/option that tells the xdelta3 command to decompress
-s specifies the source file

For example mine was this: .\xdelta3.exe -d -s ".\Kidou Senshi Gundam - Gihren no Yabou - Axis no Kyoui V (Japan, Asia).iso" .\EBOOTPatch ".\Kidou Senshi Gundam - Gihren no Yabou - Axis no Kyoui V (Japan, Asia) Patched.iso"

.\ means the local directory that you are currently working in. Since I'm in the directory with all three files, I use .\ to let powershell know the path is to the files in my current directory.

If you're unsure of what directory you're in use the pwd command.