Skip the next paragraph for the actual tutorial! Its just a boring story any ways.
Original Firmware 4.2.1. Link for any original firmware Here!
Custom Firmware 4.2.1 w/ baseband 6.15 Link via mediafire- http://www.mediafire.com/?e0xcxzpf1pagd
You also need redsn0w! Which you can get here!
oh! and obviously iTunes!
First step download the custom firmware, redsn0w and iTunes.
Second you have to put iPhone 3g to DFU mode. Instructions Here!
Third open up redsn0w and browse for 4.2.1 original firmware.
Fourth click on PWN DFU MODE! Wait.......
Fifth Open up iTunes, your iPhone should show up.
Sixth Hold Shift on your keyboard while pressing restore on iTunes. Wait......
Seventh when it is done open cydia and do updates, then you can search within Cydia, "Ultrasn0w" to unlock your iphone 3g w/ baseband 6.15.
Enjoy!