here is the guide straight from greenpois0n:
How To Use GreenPois0n: The Quick Guide
Remember: greenpois0n is not an unlock, it is a jailbreak! (If you are unsure of the difference, stop reading now & become friends with a revolutionary new website called Google.)
Download & extract greenpois0n from the .zip file, then launch greenpois0n.
Attach your iPhone, iPad or iPod to your computer & click ‘Jailbreak’.
Greenpois0n will automatically commence its DFU guide. If you know how to put your device into ‘DFU Mode,’ you can do this at your leisure; when done, greenpois0n will acknowledge a successful DFU operation.
Press ‘Jailbreak’ again (if necessary) and let greenpois0n do its thing! You will see wonderful lines of text scroll on your device’s screen to assure you that something is happening.
After a few magical seconds, greenpois0n will display a ‘Complete’ message. At this point, the greenpois0n injection is complete – just wait for your device to reboot automatically.
Once your device has finished rebooting, double-check that you have an active internet connection, then locate & press the ‘Loader’ icon on your device. This action will download the Cydia app and install it for you.
When Cydia has been installed successfully, your device needs to (& will automatically) reboot one final time.
You’re done! Enjoy
Camera roll will be empty if the jailbreak is incomplete. Rerun the app once to fix, and then another time to complete jb.
If the Absinthe webclip shows “Error establishing a database connection”, please go to Settings, turn on VPN and wait instead.
The VPN connection will give an error. This is normal. A reboot should happen a few moments after the error.
Just reboot and redo the jailbreak if you have problems.
When jailbreaking: Do not have a passcode set. Do not have your own VPN setup (Absinthe needs to setup its own).
If Absinthe crashes, reboot your DEVICE and try again.
Toggle VPN only AFTER Absinthe says it’s done, or it will not work. VPN SHOULD error and then reboot soon. If it does not, rerun Absinthe!