Update iPhone 3GS version 4.1 to 5.1.1

Hello, I have iPhone 3GS unlocked and jailbroken with limera1n Here are the statistics of my device: Version: 3GS 4.1 Baseband: 06.15 Bootrom: Old SHSSH: All shssh are present in Cydia server. (queried and confirmed) I want to upgrade it to version 5.1.1, please suggest Do's and Don't which I need to follow as I am on old bootrom. Thanks Shikhar