This method can be used by going to Restore/Downgrade, selecting your preferred iOS version, and enable both Jailbreak Option and Hacktivate Option. This method of hacktivation is only available for iPhone 2G, 3G, and 3GS devices.
This method uses an iOS 6.0 jailbreak with the iOS 6.1.6 filesystem, so it could cause some unknown issues. You can follow these step-by-step instructions to jailbreak your iPhone 3GS and iPod touch 4G on iOS 6.1.6:
I am currently trying to jailbreak my iPod touch 4g on 6.1.6 with a Windows 7 VM (as I am on Arch Linux (but I do have a Windows PC, but THAT never worked either.)), so I'll tell you how that goes.