If you want to jailbreak your iPhone 3GS device with iOS 4.2 software then probably the simplest way to accomplish that jailbreak is by using PwnageTool which will jailbreak your iOS 4.2 running iPhone 3GS.

But hot do you go about it? Well Steven Finch over at Crenk has come up with a 12 step how to guide to jailbreaking only iPhone 3GS handsets with the old bootrom still supported, and advises that you proceed at your own risk.

So here we go… first off you’ll need to download PwnageTool 4.1.2, next download the newest version of PwnageTool Bundle for iPhone 3GS on iOS 4.2 and extract to the desktop. Next download iOS 4.2 Gold Master for iPhone 3GS via doing a Google search if you are not an iPhone developer.

Now that you have downloaded all the stuff, step 4 needs you to copy the downloaded PwnageTool to a folder on your desktop and then “right click” and click on “Show Package Contents.” Next navigate to “Contents/Resourses/FirmwareBundles/” then paste the “iPhone2.1_4.2_8C134.bundle” file and close folder.

Haft way there so now move atvBundlePatcher.sh along with Info.plist.patch to the same directory as PwnageTool. Rename atvBundlePatcher.sh to atvBundlePatcher.command and then double click to allow Terminal to execute the script.

When done close all windows and start the patched version of PwnageTool and select “Expert Mode,” then browse for original iOS 4.2 for iPhone 3GS firmware. Then select “Build,” and click next and PwnageTool will now create a custom .ipsw file which is jailbroken for your iPhone.

You now need to enter DFU mode so hold power and home buttons for ten seconds, release power but still hold Home for a further ten seconds and you will now enter DFU mode. Next restore the iPhone 3GS with custom firmware 4.2 file.

Star iTunes, click on your handset in sidebar, press and hold left “alt” button or “Shift” on Windows keyboard, now click on “Restore” in iTunes but not “Update or “Check for Update”, and release the “Shift” key which will make iTunes prompt you to select the location of you custom 4.2 file, then select the custom .ipsw and click “Open.

The final step is basically just waiting for iTunes to do its thing which will involve several automated steps, and all you need to do is wait until iTunes installs the new firmware 4.2 to your iPhone 3GS and once your device has restarted it should be a fully jailbroken iPhone 3GS on iOS 4.2.

So that’s it, job done, so if any of our iPhone 3GS toting readers give this a go feel free to let us know how you get on, but remember tampering with your device can cause serious problems and thus you do so at your own risk.