In following you can find step-by-step guide to learn how can you unlock iPhone 4, and iPhone 3GS over jailbroken iOS 5.1 using UltraSn0w Fixer.
How to Unlock iPhone 4, 3GS on iOS 5.1
Tap on “Cydia” from Springboard, and then tap “Manage” tab
Tap “Sources” > “Edit” > and “Add” button and type the following source in field and tap “Add Source” button
Tap “Search” tab and search for UltraSn0w Fixer, and when found tap on that to install
Now search for “UltraSn0w” through “Search” tab and install it and reboot
Done! You have successfully unlocked iPhone 4, 3GS over jailbroken iOS 5.1 with UltraSn0w Fixer.