The HellKat custom ROM is not an official release by Samsung, but just a custom ROM that would require a custom recovery like ClockworkMod to flash the ROM zip and Google Apps zip files on the Galaxy S3.
Users need to have proper backup for their personal data on the phone as they will have to wipe all the previous data and apps off before flashing the custom ROM. In addition, the handset should have at least 80 percent of battery power, and the computer to be used in the installation process must have the USB drivers for the Galaxy S4 installed. The users also need to ensure that USB Debugging is enabled on the phone.
During the installation, if the device gets stuck on booting, users can perform wipe cache partition and wipe dalvik before rebooting the phone. Here is the tutorial to install Android 4.4.2 KitKat on T-Mobile Samsung Galaxy S3 using HellKat custom ROM:
Note: International Business Times cannot be held responsible if anything goes wrong. Users should proceed at their own risk.
Step 1: Download HellKat Android 4.4.2 ROM and Google Apps for Galaxy S3 to your computer.
Step 2: Connect the Galaxy S3 to PC via USB and copy the HellKat ROM and Google Apps packages to the root folder of your handset’s SD card without extracting them.
Step 3: Disconnect USB and turn off your phone.
Step 4: Get into the Recovery Mode. To do this switch on the phone while pressing and holding Volume Up, Home and Power buttons together.
Step 5: In the Recovery Mode, perform a factory reset by selecting “wipe data/factory reset.” Use the Power button to select options. Now, perform a cache wipe by selecting “wipe cache partition.”
Step 6: Now wipe the dalvik cache as well. From CWM recovery, go to “Advanced Menu” and then select “wipe dalvik cache.”
Step 7: Go back to the main recovery screen and select “install zip from sdcard” and then select “choose zip from sdcard.”
Step 8: Use the Volume keys to go to the Android 4.4.2 ROM zip file copied to the SD card and select it using the Power button.
Step 9: Repeat Step 7 and Step 8 to install Google Apps.
Step 10: Once the installation is complete, press the “+++Go back+++” button to go back to the main recovery menu and select “reboot system now.”
The phone will now reboot, a process that will take about five minutes. Do not press any button during this period.
[Source: Team Android]