TaiG is a tool for Windows that provides untethered jailbreaks for all devices on 8.0 through 8.1.2.
TaiG runs on already-upgraded
un-jailbroken devices.
On Twitter: @TaiG_jailbreak.
Steps: Read the instructions (tutorial) on TaiG's website. Make sure your computer has iTunes installed. Download TaiG. Open it, plug your device into your computer, and follow the instructions.