Firstly check out the video guide and then we will start the guide:
NOTE: If you are going to use a Windows virtual machine on your Mac you will have to download iTunes in order to recognize your device.
STEP 1: Update your iPhone to iOS 8.1.2 for iPhone/iPad or iPod Touch, you can download it from here.
STEP 2: Connect your iPhone to your computer and make sure to disable both Find My iPhone and passcode.
STEP 3: Download TaiG 1.2 for Windows.
STEP 4: Once it is installed on your computer, open it and the program should detect your iPhone. Once it does, uncheck the 3K Assistant app install option, and click the green Start button
STEP 5: Wait until the jailbreak completes and close the TaiG tool when prompted
STEP 6: That’s it, now you can open Cydia store and enjoy your new jailbreak..
Have you jailbroken iOS 8.1.2 yet?