Dev-Team Releases Unofficial PwnageTool Bundle for iPhone 3GS

Update #3:  For those Mac users with an old-bootrom 3GS who really know what they’re doing, here’s a minimal 3GS 4.2.1 bundle that will get you to 4.2.1 without updating your baseband.  Be sure to uncheck “Activate the iPhone” using Expert mode.  To actually jailbreak after you’ve restored with the help of that bundle, please use redsn0w.  If you don’t know how to drop a bundle into, please hold off on 4.2.1 until it’s untethered for everyone
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How to add Firmware Bundle to PwnageTool

Before I tell you some instructions on how to add a firmware bundle to will have to add the unofficial iPhone Dev-Team bundle that will let you to create a custom ipsw for the old bootrom iPhone 3GS that won't update your baseband when upgrading to iOS 4.2.1.

Here's some easy steps just follow me :

STEP 1 : First create a folder on your desktop called "Pwnage" then download Pwnagetool 4.1.3 from here into the folder after downloading You will also need to get the minimal iPhone 3GS bundle from here. Double click to extract it to the Pwnage folder.

 STEP 2 : after that double click to mount PwnageTool then drag the PwnageTool icon into the Pwnage folder.

 STEP 3 : Then right click to extract pwnagetool app after that choose show package contents from the menu that apper.

STEP 4 : Then navigate to Contents/Resources/FirmwareBundles/

STEP 5 : After that copy and paste the iPhone2,1_4.2.1_8C148a.bundle from the Pwnage folder on your desktop into the FirmwareBundles folder.

 STEP 6 : Almost done now launch PwnageTool from the Pwnage folder on your desktop and the application will detect your new firmware bundle.