Step One
Create a folder on your desktop called Pwnage
Download RedSn0w from here and place it in the Pwnage folder. Likewise, download the latest 4.2.1 firmware from below and place it in the same folder.
4.2.1 (3G): iPhone1,2_4.2.1_8C148_Restore.ipsw
Extract the redsn0w executable from...