help trying to upgrade iphone 3g 3.1.3 to 4.2.1??

[alttuned]

Hey everyone, i reallly need some help.

RIght now i am on an unlocked iphone 3g, firmware 3.1.3 and baseband 5.12.01. They guy who i initially got the phone from used jailbreakme.com.

I was wondering how to upgrade to 4.2.1, but not upgrade the baseband so i can keep the unlock and jailbeak. Can someone walk me through how to safely upgrade?

I know that once i have 4.2.1 installed i can use redsn0w and the 4.2.1 ipsw to jailbreak and unlock the phone. I just don't know how the baseband works really.
All help is appreciated

EDIT: so appearantly i have found that it is better to use Sn0wbreeze 2.2.1 to create a custom firmware to retain the unlockable 5.12.01 baseband, but i will run into an error after the itunes reboot, and need to use fixrecovery to kick it out?

Is this method recommended? i dont want to mess up the iphone with the error...

[iYeow]

Yes, definitely snowbreeze is the way to go to preserve your baseband. Use ireb to put your iphone into pwned dfu mode and then restore to custom firmware of 4.2.1

You must have atleast itunes 9 or 10 too