The iPhone Dev Team has updated Redsn0w to include support for iOS 5.1.1. With this update, you are no long required to “trick” Redsn0w into Jailbreaking 5.1.1 by pointing it at the iOS 5.1 IPSW file. This is still a tethered jailbreak and I would recommend staying on a lower firmware if possible.
Note: This only works for A4 devices (iPhone 4, iPod Touch 4G, iPad 1) or older (iPhone 3GS, iPod Touch 3G). This means that this jailbreak will not work on the iPhone 4S, iPad 2, or iPad 3.
Step 1: Download and open RedSn0w 0.9.11b1
Step 2: Click “Jailbreak” and put your device into DFU mode (as demonstrated in the video above)
Step 3: Click “Install Cydia”
Step 4: After the process is finished, wait for your device to reboot
Step 5: Close RedSn0w & re-open it
Step 6: Go to Extras > Just Boot & put your device into DFU mode once again