How to go from — Locked iPhone 3GS 4.2.1 baseband 05.14.04 — to — Unlocked iOS 6

A friend of my gave me his old iPhone 3GS. It is locked to a different carrier than mine, it has no SIM card from the carrier it is locked to (my friend is using it in his new phone). It has iOS 4.2.1 installed and the baseband is 05.14.04 My goal would be to get it unlocked and to have iOS 6 (or the latest possible) installed and working. From what I have read in my research around the web, it seems that I may have to make it go to an iPad 1 baseband 06.15.00 (which scrambles GPS and WiFi) and only then I can downgrade the baseband (this part, I guess, using redsn0w 0.9.15b3) to something more useful that gets me to an unlocked iOS 6 (unthetered) state. — Can anyone help me on how can I do that (bear in mind the phone has no valid SIM)? — What are the steps to do it right? Does it all can be done using redsn0w 0.9.15b3 ? It has been quite a few months waiting for the baseband downgrade, but still couldn't find a clear answer of how to unlock and upgrade to latest "unlockable" iOS 6, from a locked iOS 4.2.1 3GS phone. Can anyone help me on this? Thanks.