This video tutorial will teach you how to jailbreak and unlock iPhone 3G on IOS 4 frimware using Redsnow. The first thing that needs to be done is to download Redsnow and you also need to download IOS 4 official firmware file.
Jailbreak and unlock iPhone 3G on 4.0/4.0.1 – iOS4 Final
Files need to download
To unlock your iPhone 3G add this source to Cydia and Install Ultrasn0w: repo666.ultrasn0w.com