Yes, you need the modded ODIN. Also, unzip the firmware file once it's downloaded. In it should be an AP, CP, CSC, Home_CSC and BL files.
- Ensure your phone drivers are installed.
- Turn the phone off
- Boot your phone into Download mode.
- Open Modded ODIN
- Connect USB cable. Once your PC recognizes the phone, ODIN will show COM0 (I think COM0) near the top left corner.
- Click on BL button, navigate to the BL file from the download and select it.
- Do the same for AP and CP files.
- For CSC, select the CSC file from the download, NOT the Home_CSC. Pretty sure you leave the USER_DATA blank, although I always second guess myself when trying to help others.
- Once you have the 4 files selected, hit start. It'll take a few minutes for it to transfer and flash the files..