Watch video at 
http://www.youtube.com/watch?v=UlvFE3bj118

Greenpois0n works for:
iPhone 4
iPhone 3GS
iPod Touch 4G
iPod Touch 3G
iPod Touch 2G
iPad

Does NOT Work for:
iPod Touch 1G
iPhone 2G
iPhone 3G

Simple Steps
1. put in DFU MODE
2. open up greenpois0n and click jailbreak
3. connect to wifi on device
4. look for blank app called loader on ur    device and click it. 
5. click and install cydia
 and when done remove loader.


Detailed steps (if u want a complete guide)
1. Get greenpois0n from site and choose ur     machine. http://www.greenpois0n.com/

2. Extract greenpoison and put ur device in    DFU mode. to do this simply hold the        sleep and home button until the screen      goes completely blank (8secs) then          immediately release only the sleep          botton until u see the cord on the          screen (8secs). You are now in DFU Mode.

3. Open up greenpoison and click "Prepare      to Jailbreak"

4. Once done successfully read the             instructions carefully so u can know        what to do. when ur done click reset and    the timer will count u down at each         step. IF U THINK U MESS UP AT ANYTIME       JUST CLICK RESET AGAIN AND START OVER.

5. When u reach at the last step keep          holding down the home button until u see    jailbreak complete.

6. When done u'll see a blank app on    ur     device. Ensure u connect to wifi            before clicking it. this will install       cydia and jailbreak complete. 

7. Once cydia is on it may ask u to delete     the loader blank screen u can safely do     so.

Thanks to Derron. "Making things easier for you!"