Things you'll need. iDevice with iOS 4.1 (Stock) USB cable iTunes max on 10.0.1.22 redsn0w for windows v0.9.6b2 (Download link) Stock firmware for your device (same as your current firmware which is 4.1) Custom firmware for your device (4.1) Things used in this tutorial iPhone 3GS on iOS 4.1 iTunes 10.0.0.68 Windows 7 x86 Using redsn0w and entering DFU mode Step 1 Download and run redsn0w.exe The home screen looks like this Please Register or Log in to view images Click Browse and browse to the Stock firmware and select it. It will ask you if your iPhone 3GS is new model or old one (other device users can skip it as you won't see this message) Click yes if you have iPhone 3GS MC (new) model or No if you have non MC (old) model. To identify if you have MC or Non MC model, go to Settings - General - About and scroll down to Model. If it's MCXXXX then you have new (MC) model. If it's something like MBXXXX, then you have old (non MC) model. Please Register or Log in to view images Step 2 Click Next Now redsn0w will apply kernel patches Please Register or Log in to view images Please Register or Log in to view images Step 3 redsn0w will now display 3 options. 1. Install Cydia 2 Just boot tethered right now 3. Just enter pwned DFU mode right now. Please Register or Log in to view images Select the third option. Click next Step 4 Read the message on the screen carefully. Connect iPhone to PC if not done already. Power Off iPhone and click next. Please Register or Log in to view images Step 5 Now follow the on screen instructions. You must follow them correctly in order to enter DFU mode properly. Please Register or Log in to view images If you didn't do it right, it will show you message that you probably misunderstood the instructions. If so, click OK and start again. Please Register or Log in to view images When you'll get in DFU correctly, it will start process again. Once done, it will show message that your device is in pwned DFU mode. Please Register or Log in to view images Click finish Please Register or Log in to view images Step 6 Open iTunes, It will say that it has detected iPhone in recovery mode. Click OK. Now Hold SHIFT key on your key borad and click the Restore button in iTunes. Now release the SHIFT key and browse to the custom IPSW file and select it. iTunes will start process of restoring your device to that firmware. This process is same as other restore process and once done, you will get Jailbroken iPhone. If you downloaded Activated IPSW, then your iPhone will be activated as well. NOTE: People are asking if this method keeps the baseband. Please note that this is not the tool you are looking for. If you want to keep your current baseband, and if you have iPhone 4, you can use latesr TinyUmbrella. If you are on other devices, you can get a custom firmware cooked with PwnageTool with having "Preserve Baseband" option checked. You can tell a friend of yours to create a custom firmware for you, or you can search for it. [Source] I take no credit for this tutorial.