First and foremost, it is important to note this is a tethered jailbreak and is only meant for devices boasting an A4 processor (or below), namely: the iPhone 4, iPhone 3GS and the 4th-gen iPod touch. Devices with A5 (iPhone 4S, iPad 2), A5X (iPad 3) and A6 (iPhone 5) processors are not supported at this point. Hacktivation + baseband preservation support has also been added for iPhone 4 and 3GS.
Instructions on how-to jailbreak iOS 6:
2. Download iOS 6 from here, and update your A4 powered device to it. Those of you who rely on unlock please read the note below before proceeding with the update.)
3. Start Redsn0w. Run it in Administrator mode if you are on Windows. OS X Mountain Lion users need to start it by Ctrl-Click on the Redsn0w icon and then selecting Open from the new menu.
4. Click on Jailbreak, select Cydia and enter DFU mode when Redsn0w prompts you to with the help of onscreen instructions. Once the device is in DFU mode, Redsn0w will start jailbreaking the device using limera1n exploit. When done, select “Autoboot this device when it connects in DFU mode” option to enter tethered mode for the time being.
After a short while, the device will reboot with familiar Cydia icon on the home screen.
Note for iPhone 4 and iPhone 3GS Ultrasn0w unlockers: You will first need to preserve your older baseband to be eligible for unlock. Click on “Extras” option and then select “Custom IPSW” option. Select the required iOS 6 IPSW file for your device, and let Redsn0w make the custom firmware for you. Once done, enter Pwned DFU mode by selecting “Pwned DFU” from Redsn0w’s Extras option, and then restore to this custom firmware.
How to boot your device tethered on iOS 6:
Since this is a tethered iOS 6 jailbreak, you will need to boot your device in a tethered mode every time the device is turned off and back on. Booting in tethered mode is easy, and can be achieved using Redsn0w: click on the “Extras” from the main screen and then simply click on “Just Boot” option. Follow the on-screen instructions. Once done, you will be rebooted back into tethered-jailbreak mode.