If you are looking to unlock your Apple iPhone 3G with the iPhone Dev Team’s yellowsn0w solution yellowsn0w is fairly straightforward, just remember when tinkering around with baseband you run the risk of ending up with a paperweight rather than an Apple iPhone.
Yellowsn0w needs the iPhoner to update the iPhone 3G baseband firmware to 02.28.00 firmware which comes bundled with the iPhone 2.2.OS.
But maybe you are not too up on all this unlocking gear and would like a simple to follow guide to getting it done, if so then the guys over at Intomobile have come up with an 9 point walkthrough to assist in the unlocking process so follow the source.
Source — Intomobile