First of all you need to be jailbroken, but this is not covered here.
Go into Cydia and click "Manage Sources" then "Edit"and add: repo666.ultrasn0w.com
Then search for "ultrasn0w" as a package and install it, that's all there is to it, your iPhone is finally unlocked so when you travel abroad you can avoid ridiculous roaming charges, or you can switch providers and use any SIMcard you like.
You'd really thi........