Before you jailbreak your device perform the following steps:
1. Back up your iPhone with itunes
2. Download TinyUmbrellaGo.
a. Go to TinyUmbrella here: http://thefirmwareumbrella.blogspot.com/
b. You will download the version for your system, either OSX for Mac useres, or the WIN
version for Windows users. Look for this box for your download:
c. Plug in your iphone and open TinyUmbrella until it is recognized. The program will guide you
through the process. Basically, you will click a button to "save shsh" and the program does
the work for you.
***This TinyUmbrella stage is important to back up your SHSH blobs possible for future
jailbreaks, but more importantly TU has an important feature if your iPhone is ever "stuck" in
recovery mode. There is a "kick out of recovery" button that has saved me before.***
3. Go to http://www.greenpois0n.com/ and download the software for your operating system for the
Greenpois0n jailbreaking tool for iPhones with 4.2.1
4. After the software is installed, click on the Greenpois0n icon to run the program. It should look like this:
5. The screen will instruct you to power off your device, and then connect it to the computer with
a usb cable.
6. Press the "prepare to jailbreak (DFU)" button. At this point the screen on your computer will guide you through a series of steps of pressing the home (circle button at the bottom of your phone) and sleep (the button on top of your phone) buttons. If you don't follow the sequence properly, don't worry, it gives you the option to try again and again if needed. The sequence is:
*Press and hold the sleep button for 2 seconds.
*Continue holding sleep and press and hold home for 10 seconds.
*Release sleep button and continue holding home for 15 seconds.
You don't need to memorize this because the instructions will be on the screen with a countdown timer to help you out!
7. Once you've successfully entered DFU mode, the jailbreaking begins. It takes some time and there will be some tiny writing and scrolling on your iPhone screen. Just let it do it's work.
8. When it has finished with this, your device reboot and have a greenpois0n animation instead of the apple that you're accustomed to seeing. You're now jailbroken! There will be a new icon on your home page called "loader." Open this and it will tell you how to install Cydia, which is the jailbroken app store. Install Cydia, reboot, and have fun!