In other words, make sure to install a Jelly Bean kernel first, before installing any Jelly Bean ROM.
If Status 7 error appears on screen, it could be due to an incompatible device being flashed or a related kernel might be missing.
Allow the newly installed ROM to stabilize for about five to ten minutes before you start working on it.
Verify that the device has at least 80% battery charge to prevent unexpected device shutdown (due to battery drain) during firmware installation.
Ensure the device is rooted with ClockworkMod recovery (CWM) installed on it.
Then select the checkbox to enable USB Debugging mode under settings > Developer Options.
Tap the Build Number seven times until Developer Options is unlocked or revealed under Settings.
Navigate to the bottom of the screen and select Build Number.
Otherwise if device is running Android 4.2, then do the following steps:
Enable USB Debugging mode on the phone by navigating to Settings > Developer Options (if device is running Android 4.0 or later) or go to Settings > Applications > Development (if phone is running Android 2.3 or earlier).
CYANOGENMOD 10.1 DOWNLOAD GALAXY S3 DRIVERS
Make sure the correct USB drivers are installed for your device on the computer ( download Galaxy S3 GT-I9300 USB drivers).
Backup your important data and settings on the phone to avoid data loss while performing any data wiping task during or after firmware installation.
Verify the device's model number by navigating to Settings > About Phone. Do not attempt this installation on any other device, as it may inadvertently brick or damage the device.
CyanogenMod 10.1 Nightly ROM and the instructions provided in this guide are limited tothe international variant Galaxy S3 GT-I9300 only.
The CM10.1 ROM is based on original Android Open Source Project (AOSP) release, which brings the latest Android 4.2.2 Jelly Bean features powered with superior performance and greater stability than stock Android versions. Samsung Galaxy S3 GT-I9300 users can now enjoy a rock stable Jelly Bean experience on their device, thanks to the release of popular CyanogenMod 10.1 Nightly ROM by the industrious CyanogenMod team. Install Android 4.2.2 Jelly Bean Based CyanogenMod 10.1 Nightly ROM on Galaxy S3 I9300