C:/Users/ *username* /AppData/Local/Temp/ sn0wbreeze and paste the bundle zip there, keep sn0wbreeze open. (don't unzip it, put it just how it is. If there is no AppData folder, open a folder, go to tools, Folder options, then go to View tab and select Show hidden files, folders and drives).
Now you can select your IPSW, and make the IPSW. After that, open iREB RC2 (you have to download from ih8sn0w's site) and enter pwned dfu. Don't use the DFU Helper inside sn0wbreeze, it probably won't work. After entering pwned dfu, open iTunes and Shift+Restore to the custom IPSW.
WARNING! This has not been tested yet. Use at your own risk.
Jailbreak can be sucessfully done using Redsn0w 0.9.6b4, PwnageTool 4.1.2, Sn0wbreeze 2.1
Download 2
Cydia Patch for IOS 4.2.1 GM
copy it to /Applications/Cydia.app/Cydia_ by SSH’ing into your iOS device. [NOTE: You must have SSH enabled on your device]
Now by using the original Cydia binary from here, type the following commands in Terminal app on Mac OS X:
mv Cydia_ Cydia.bak
bspatch Cydia.bak Cydia_ Cydia.patch
chmod +x+s /Applications/Cydia.app/Cydia_
Cydia Patch for IOS 4.2.1 GM
copy it to /Applications/Cydia.app/Cydia_ by SSH’ing into your iOS device. [NOTE: You must have SSH enabled on your device]
Now by using the original Cydia binary from here, type the following commands in Terminal app on Mac OS X:
mv Cydia_ Cydia.bak
bspatch Cydia.bak Cydia_ Cydia.patch
chmod +x+s /Applications/Cydia.app/Cydia_
[via: SinFulIPhone]