1. Download Odin from this post.
2. Download I9305_CWM_Recovery.tar.md5
3. Unzip Odin to your desktop.
4. Copy the I9305_CWM_Recovery.tar.md5 to the same directory.
5. Boot your SGS in to download mode.
(Power off and then press and hold the power + volume down + home buttons at the same time)
6. Connect your phone to your PC using the cable that came with it.
7. If Kies is running on your pc shut it down.
8. Open the Odin application.
(Make sure your phone is detected, there will be a yellow square in the top left corner.)
9. Choose PDA button and select the I9305_CWM_Recovery.tar.md5.
10. Press start and wait. Your phone will be flashed and reboot when it is finished.
(After Odin finish let the device reboot to android at least one time before applying I9305_CWM_Recovery.tar.md5.)
11. When the phone reboot, power it off again.
12. Put the phone in recovery mode.
(Hold down power, volume up and home when the device is turned off and unplugged.)
13. After that you can flash the Root_SuperSU.1.02-Busybox.1.21.0.zip to be ROOTED to any firmware and make a backup and restore of your firmware, also you can now flash any custom rom / kernel made for GT-I9305.
Many thank's to Chainfire for the CWM recovery and also to mick.wheelz for the successful test's of my ported recovery.
To reset the counter back to 0 use the TriangleWay from Chainfire.
1). Copy Root_SuperSU.1.02-Busybox.1.21.0.zip and Remove-boot-recovery.p.zip to your device internal memory card.
2). Boot your device in to download mode.
(Power off and then press and hold the power + volume down + home button at the same time)
3). Flash I9305_CWM_Recovery.tar.md5 with Odin with unchecked the Auto Reboot box.
4). After Odin finish do not turn off or restart the device but try to go in recovery mode.
(Press and hold the power + volume up + home button at the same time)
5). Once in recovery, flash the Root_SuperSU.1.02-Busybox.1.21.0.zip to gain root and Remove-boot-recovery.p.zip to delete the boot-recovery.p file and then reboot.