Thus, for unlocked iPhones that are using ultrasn0w it's not recommended to jailbreak using the just released version of Redsn0w.
How to Jailbreak iOS 4.1 For iPhone 3G and iPod Touch 2G using Redsn0w 0.9.6b1 For Mac1. Download iOS 4.1 according your used machines. And then update your firmware using IPSW file you downloaded from iTunes 10
2. Download redsn0w 0.9.6b1 from this link.
3. Now launch the redsn0w you downloaded from the step 2, and then direct it to iOS 4.1 in order to start jailbreaking.
4. You need to install Cydia and of course you can also install other options on this phase but is not mandatory except Cydia. And then click "Next"
5. Before click the second "Next" button after the above step, be sure your device is both OFF and plugged into your computer.
6. You will be prompted to put your device into DFU mode. To do that, hold Home + Power button together, wait for few seconds after that release the Power button, while you still pressing the Home button.
7. Redsn0w will complete the further and necessary process to jailbreak your iPhone. Once it's done, it will doing the automatic restart for your iPhone. And your iPhone is jailbroken with iOS 4.1 firmware within it.
The above steps can be used to jailbreak iOS 4.2 Beta too.