Jailbreaking does not brick your device, any errors can easy be corrected using the restore function, so I feel it safe. Now for this test I was using an iPod Touch 3G 8GB. Other similar tutorials miss out on an important step which is the reason you can jailbreak but I’ll explain in detail.
First thing you need to know is SHSH blobs, you dont need to know the technical details, just need to know that you need these to be able to downgrade to a lower version. This is why most iPhone iOS 4.02 tuorials work just for a few people. If your SHSH was saved on Cydia (ie you had jailbroken the device before), the downgrade would work while others would get a 3002 error. This tutorial will work for everyone, including those who dont have their SHSH saved on Cydia.
First we need to download TinyUmbrella to view some details about the device and trick iTunes to allow you to downgrade versions. Download TinyUmbrella for your operating system. Right click on the downloaded file and “Run as administrator”. Connect your device using the USB cable. Now click on the save SHSH button. This will save your SHSH blobs to Cydia and you will get a confirmation.
Now start the TSS server by clicking the button, this is actually supposed to trick iTunes by sending the verification request to a dummy server. The software however does not work correctly, so we are manually going to change the address values.
Open My Computer and navigate to C:\Windows\System32\drivers\etc\
Here you will find a file named hosts. Unfortunately Vista and Win 7 users cannot modify the file directly. So copy this file to your desktop. Now open the file using Notepad (open Notepad, drag and drop the file into it).
Change the line 127.0.0.1 gs.apple.com to 220.127.116.11 gs.apple.com
Save the file and copy it back to its original location (C:\Windows\System32\drivers\etc\)
Now we need to download the firmware iOS4.0 for your device. Download iOS 4.0 firmware from felixburns.de. If you dont know which one you need, just enter the last 3 digits of your iPod or iPhones serial number and it will tell you what device you have. Download the corresponding firmware 4.0 for your device.
Connect your device to iTunes, while still connected to iTunes turn off the device by holding the power key and then sliding the power off option. Once the device is turned off we are going to restart it in Restoration mode.
Simply hold down the power and home button of your device together and it will start in restore mode. The device will boot up into restore mode. It will look like the image below. Incase you didnt press the buttons correctly, shut down and do the step again.
Itunes will now give a message that the device was found in Restore mode, click OK.
Click the Restore button on iTunes while holding down the Shift key. It absolutely important that you click it while pressing the Shift key on your keyboard.
Now you can choose the firmware you wish to install to your iPhone or iPod. Select the firmware you downloaded earlier and click OK. The device will now install the firmware and reboot.
Now the easy part, activate Wifi and start Safari. Go to www.jailbreakme.com on your device and follow the instructions (slide to jailbreak).
Congragulations your iPod iOS 4.02 is now jailbreak ed. You should see Cydia installed on the device. Have good time with it.
Just wanted to add that Cydia has its custom sources for getting additional apps but I’m not going to cover that in this tutorial.