UPDATE: Apple released iOS 6.0.1, so you can NOT restore to earlier firmware (both original and custom), unless you have saved your SHSH keys. If you are on iOS 6.0 right now you can use this tutorial.
This is step-by-step instructions on how to perform tethered jailbreak of youriPod Touch 4G on iOS 6 firmware using RedSn0w 0.9.15 for Windows.
Tethered jailbreak means that each time you restart your device you will need to connect it to a computer and run redsn0w.