Jailbreak and unlock IPhone 3GS with new bootrom

I do not own an iPhone but had the opportunity to unlock the iPhone 3GS recently. I tried looking online for information on unlocking the iPhone. Unfortunately, there is only a mess of information online and no single clear guide on how to accomplish this. With some patience, I was successful at unlocking it. Here is the requirements and steps to do so.

Specifications

• The iPhone is a 3GS running iOS 4.2.1 (8C148a) with the 05.15.04 baseband. You can see the version and the baseband from the Settings->About page.
• The phone also has a new bootrom. See below for the howto to determine the bootrom specifications.

Steps

1. Jailbreaking the phone [Reference]
In order to unlock the phone, the phone must have the 06.15.00 baseband. To accomplish this, use redsn0w 0.9.6rc16 to install Cydia and the iPad (06.15.00) baseband. In the redsn0w options, select the following.

• Install Cydia
• Install iPad baseband

After rebooting the phone (via tethered boot), if the Cydia icon is completely white, repeat the redsn0w installation.

2. Unlocking the phone [Reference]
Now that Cydia is installed (from the previous step), perform the following using the Cydia app.

• Add http://repo666.ultrasn0w.com
• Select and install ultrasn0w from the packages list

3. Enabling untethered boot on the phone [Reference] [use greenpois0n on top of redsn0w]
Unfortunately, redsn0w 0.9.6rc16 supports only a tethered jailbreak on the 3GS with the new bootrom. To make the phone support an untethered jailbreak, run greenpois0n RC6.1.

4. The phone should now be unlocked for use on any carrier.

Additional Information

To enter DFU mode from power off

• Hold the power and home buttons together for 10 seconds
• Release the power button while holding the home button for another 10 seconds

To exit DFU mode

• Hold the power and home buttons together for 10 seconds
• Press the power button to boot the phone.

To determine the bootrom version

• Run f0recast 1.3 [Download]
• Note, you must boot the phone into DFU mode (see above).

To tethered boot the phone

• Run redsn0w and follow the tethered boot steps